Canon vs Nikon... both are good.

Some previous responses are correct... D500 and 5D Mark IV really aren't comparable types of cameras. Apples and oranges... Or maybe a tool analogy such as "pliers and hammers" would be more appropriate than fruit.

Nikon D500 is a "crop sensor, APS-C" camera (as are Nikon D7200, D7500, D5600, D3400 models... Nikon calls these "DX" cameras).
Canon 7D Mark II is most comparable to D500, also a "crop sensor" camera (as are Canon 80D, 77D, T7i, T6 and SL2).

Nikon D850 is a "full frame" model (as are D750, D610 and D5... Nikon calls these "FX" cameras).
Canon 5D Mark IV is most comparable to the D810... both are 36MP models. The D810 has recently been discontinued, but is widely available.
Canon 5DS-R is a 50MP model that's somewhat more comparable to the 46MP Nikon D850, except the Nikon is a newer, just introduced model with a lot of other improvements that are more "5D Mark IV" like)
Other full frame Canon are 6D Mark II and 1DX Mark II.
D5 and 1DX Mark II are very expensive, high-performance, pro-oriented models... that you've probably already ruled out.

Your Nikon Photomic (I'm assuming an F or F2) 35mm film camera was "full frame". By definition, "full frame" digital capture an image that's 24x36mm, same as most 35mm film SLRs did. Any lenses used upon it that you still have MIGHT be usable on the above Nikon DSLRs... but those same lenses ALSO can usually be used on the modern Canon cameras via inexpensive adapters. Nikon cameras may be able to use many excellent old Nikon F-mount lenses... But, in addition to F-mount, modern Canon can utilize many DIFFERENT vintage system lenses via adapters.... something that Nikon generally cannot do. OTOH, Canon "orphaned" their own FD/FL mount lenses (discontinued prior to 1990), and those are largely not usable or practical for use on modern Canon, even though adapters exist.

Lenses "act differently" on crop versus full frame cameras. Any given focal length will behave the same as you're familiar with on FF. But due to the smaller size sensor they "act longer" on the APS-C crop sensor models. For example, while a 50mm f/1.4 lens serves as a "normal" lens on full frame/film cameras, on any of the crop sensor cameras the same 50mm lens will act as a short, fast telephoto (equivalent to "75mm on film" on the Nikon bodies, or equiv. to "80mm on film" on the Canon APS-C models... they use slightly different size sensors). The crop sensor works for you in some respects, against you in others. Telephotos are more powerful (so can be smaller and lighter). But wide angles are no longer very wide (so there have been various "ultrawides" developed, especially for use on crop cameras).

Full frame/FX cameras basically require full frame/FX lenses. Crop sensor models, on the other hand, can use both full frame/FX lenses and crop-only/DX lenses that have been developed in recent years.

Crop sensor cameras and the lenses used upon them can be lower cost, smaller and lighter weight. A typical 21MP or 24MP crop sensor camera can make excellent images. They also often have faster frame rates, faster flash sync and some other advantages over full frame. Nikon D500 and Canon 7D Mark II, for example, are top performing "sports/action" cameras... pro quality. A step down, Canon 80D and Nikon D7200 are slightly slower shooting, but slightly higher resolution. (Technically, D7200 is discontinued and replaced with D7500 that's faster, but lower resolution. D7200 is still widely avail.)

Full frame cameras generally excel when you want to shoot in particularly low light with especially high ISOs. The higher resolution models also are able to make larger prints than crop sensor cameras. These are the "ultimate" in image quality and detail, short of going to a medium format digital (Mamiya, Pentax, Fuji, Hasselblad). But to see the image quality benefits of full frame/FX, you really need to make BIG prints. For what most people print... and certainly for most online image display which is even smaller size... you will really struggle to see any difference between crop/DX and FF/FX.

Personally I use both types of cameras. Most of my shooting is sports with crop sensor cameras. I also prefer them for most wildlife and some other purposes. I use the full frame camera for landscape, architecture, some portraiture and some macro. I also used FF for low light in the past, but my newer crop sensor cameras are actually better at high ISO than my older FF, thanks to continuing improvements. Once I upgrade the FF model to a newer one, I'm sure that it will again be superior at low light work.

Let's talk about lenses....

If you get a Canon camera DO NOT buy the cheap EF 75-300mm "III" lens. It's one of Canon's worst... slower, noisier micro motor autofocus... no image stabilization... and marginal image quality, especially at the 300mm end of the zoom. You'll often see it "in kit" with various camera models, simply because it's inexpensive. Canon makes several 70-300mm lenses that are much better choices, though a little to a lot more expensive. EF 70-300mm IS USM "II" is about $450 and the latest version, with a new, advanced form of autofocus motor, image stabilization and more. EF 70-300mm "L" IS USM is premium/pro quality, costs a lot more at $1350, but is durable, weather resistant, and can optionally be fitted with a tripod mounting ring. A longer alternative that's even better is the EF 100-400mm L "II" IS USM, on sale for about $1900 right now.

The above are all Canon "full frame capable" lenses that will work fine on either FF or APS-C cameras. If you purchased an APS-C crop 7D Mark II or 80D, you'd also have choice of EF-S 55-250mm STM IS lens for about $300. EF-S lenses are designed for and can ONLY be used on the APS-C Canon cameras. They are NOT full frame compatible.

Canon uses three different types of autofocus drive... Micro motor is the slowest, noisiest and is found in the least expensive lenses. Lenses that use it aren't marked "STM" or "USM". STM is "stepper motor" and is faster, smoother running, and it's very quiet. It's often recommended for video, but is fine for all but the fastest action. USM is "ultrasonic motor" and is typically 2X to 4X faster than STM (so also far, far faster than micro motor), and is what you'll find on most premium and pro-oriented Canon lenses. USM is great for sports photography, active wildlife, as well as anything less demanding. It's quiet, but not silent, and it may not be as smooth running as STM... so may not be ideal for video. HOWEVER, in 2017 Canon introduced three lenses with a new "Nano USM", that's the best of both worlds. It's quiet & smooth like STM, plus it's fast and great tracking movement, like USM. The above mentioned EF 70-300mm IS USM "II" is one lens with the new drive. So are the EF 24-105mm f/4L IS USM "II" and the EF-S 18-135mm IS USM (the latter was the first lens introduced with it... there happen to be STM and micro motor versions of that lens too).

Finally, I've mentioned image stabilization in passing above. Using an F or F2 Photomic, you are probably unfamiliar with it. Canon pioneered putting stabilization in their SLR system lenses in the 1990s. It proved so successful that today EVERYONE offers some sort of stabilization... either in lenses (Canon IS, Nikon VR, Sony OSS) or by moving the sensor in the camera body (Pentax, Sony, Olympus). Stabilization is particularly helpful on hard-to-hold-steady telephotos, but more an more in normal to wide lenses too. The earlier forms of stabilization allowed two to three stops worth of assistance... while more recent offer up to around four stops worth. This means a 100mm lens you needed to use 1/100 shutter speed to insure a high percentage of sharp shots before... then it's stabilized you may be to hand hold with reasonable success at 1/12 or even 1/6 shutter speed, and with very good confidence at 1/25 or 1/50. Stabilization has been a real game changer... though higher usable ISO in newer cameras might seem to offset the need for it, the truth is that this just makes for even more flexibility to shoot in situations you never dreamed possible, back when you were shooting Ektachrome 100 and Velvia 50 with those old, manual focus lenses and clunky SLRs.

DO NOT scrimp on lenses. They're more important deciding the qualities of your images, than the camera they're used upon. ALL current DSLRs are quite capable... even the cheapest, most entry-level models. You'd be wise to spend less on the camera, more on the lenses.

I've used Canon examples above because that's the system I use and know best. There are similar choices among Nikon, I'm sure.

Nikon and Canon offer the most choice of DSLR models, as well as the broadest system of lenses and accessories to use upon them. For example, they each have about 90 lenses to choose among. I know Nikon has some limitations and it's a good idea to check compatibility charts on their website or elsewhere online, to be certain a lens under consideration will work on a particular camera. Aside from EF-S lenses only being usable on their APS-C cameras, there are no issues with Canon lenses and cameras made the last 30 years. Even the oldest EF lens will work just fine on the newest DSLR model.

Pentax, Sony, Olympus and others offer some excellent digital cameras too... but their systems just aren't nearly as comprehensive.

Third party manufacturers such as Tokina, Tamron and Sigma... as well as flash, etc. makers... also make the most items for Canon and Nikon... which they may or may not offer for other systems.

What lenses you might want depend on:

1. What you want to shoot and how you want to shoot it.
2. Whether you'll be using a crop sensor camera, full frame camera, or both.
3. How much you want to spend and how heavy gear you're willing to carry around.

Without knowing more of your particular plans and preferences, it's hard to recommend much beyond the generalities described above.

I had a D850 on preorder for an extended period and finally cancelled the order. I have a Sony A6000 and A6300 with APC Sony G & Zeiss Glass which I found myself using frequently and led me to start seriously exploring the Sony A7RIII on YouTube and other sources... I purchased an A7II to see if it was something I would really like and I do. After a lot of internal struggle, I will be purchasing an A7RIII in January instead of a D850. I'm heavily invested in Nikon Cameras, Lenses and Gear, however, it looks like I will be adding some more Full Frame Sony and Zeiss Glass. I plan on keeping my D810, DF and D500's (x2) and most of my Nikon Glass.