I think it's great that Nikon is introducing the D500.... Assuming it lives up to expectations, it will motivate Canon to really kick ass with their 7D "Mark III", and maybe sooner rather than later.
But for now all we have is comparisons of the current 7D Mark II and D500 on paper and the few differences are only marginally revealing and may or may not be particularly notable.
The D500's MSRP is $500 more than the current street price of the 7DII.
7DII has a built-in flash (capable of off-camera flash control), D500 doesn't.
D500 has an articulated LCD monitor and touch-screen controls, 7DII doesn't.
You can argue both for and against a built-in flash, articulated LCD and touch controls. These features can be nice and useful to some, but may be of little use to other people, might reduce the quality of weather and dust resistance sealing and/or effect durability to some extent.
Note the slightly larger and significantly higher resolution LCD screen on the D500... 3.2" & 2.3 million pixel vs 3" & 1 million pixel on the 7DII.
D500 has built-in WiFi/Bluetooth, 7DII doesn't. Canon takes a different approach by offering a separate Wireless File Transfer module (WFT-E7/E7A 2) with much greater operating range and faster data transfer than built-in WiFi can offer. Off course, this means spending significantly more (about $770), but might arguably be a more professional approach. Haven't yet seen any claims from Nikon regarding the D500's built-in WiFi's range... but typical in most cameras is about 40 feet or less. The Canon WFT module is able to connect roughly 10X farther away... claims upward of 400 feet. However, to be completely fair, Nikon offers a separate and more powerful WT-7A module for the D500, costing about $750 and claims it has upwards of a 600 foot operating range.
7D-series cameras inherited some design features from Canon's top of the line 1D-series.... Notably, the AF system has a discrete chip running it, while image processing is handled through dual processors. This is one of the reasons for the very high performance of the sophisticated AF systems in the 7D models (19-point in the original, 65-point in the Mk II). The 7D models also use a shutter release mechanism similar to the 1D-series, that has an exceptionally short shutter lag for very responsive shooting. We don't yet know how the D500 compares in these respects.
The CMOS sensors that Sony supplies to Nikon have pretty consistently had a bit wider dynamic range than the CMOS that Canon makes themselves (and has done for 15 years now).... at their native ISO 100. But typically when stepped up to higher ISOs the difference is gradually reduced until they are virtually the same around ISO 1600, and the Canon sensors actually tend to have a slightly wider DR at any ISOs above that. Again, there is no way to know for certain how they compare, until the D500 hits the street and can be tested.
The 7DII tops out at ISO 16000 expandable to 52100. The D500 offers up to ISO 52100 expandable to an amazing 1640000. Of course, it's questionable what's actually usable in either camera. That will vary depending upon the end purpose planned for the images being taken, post-processing and the user's expectations. For example, if planning to convert an image to black and white, it might be possible to use a much higher ISO than one would want to with a color image. It is nice to have the potential to use an ultra high ISO, if it will serve some limited purposes adequately or if future improvements in post-processing software makes it more practical to use more widely.
I don't expect the D500 will rival the high ISO capabilities of full frame cameras, any more than the 7DII does. The much larger pixel sites on most FF sensor cameras will always be superior for detail capture and lower image noise. (Note that the 50MP 5DS/5DS-R full frame models have almost exactly the same size pixel sites and density as the 20MP APS-C 7DII, but are NOT high ISO cameras by any means... However, both those FF models are capable of capturing considerably more fine image detail than 7DII... though you may not be able to see the difference unless you examine the images at ridiculously high magnifications on your computer monitor or make really big prints from each format.)
180,000 pixel metering system in the D500 versus 150,000 pixel system in the 7DII.... Best guess, there likely will be no user-detectable differences.
153-point AF system (99 cross type) in the D500 versus 65-point AF system (all cross type) in the 7DII... Well, first of all, how many AF points do we actually need? This smacks of one-up-manship that may or may not make for any real world advantage one way or the other. We'll have to wait and see how the new AF system works in the D500.
The D500 claims to have a -4EV capable AF system, compared to -3EV in the 7DII. That's approximately the difference between being able to focus in 3/4 moonlight and in 3/4 moonlight with light clouds. If true, either is a significant improvement over -1EV or -0.5EV capable systems many other cameras offer. Both cameras are "f8 capable", which is great. That makes possible some lens + teleconverter options that won't work on "lesser" cameras.
200 RAW buffer in the D500 versus 31 RAW buffer in the 7DII? Well, that's pretty impressive on paper, if it's accurate. But, frankly I try not to use burst mode more often than necessary. A single well-timed shot can often give better results than "machine gunning", which also tends to make for many more images to post-process at the end of the day... a lot more time sitting at the computer!
Besides, I'm not entirely certain the D500 actually
can buffer 200 RAW... In the Nikon preliminary info docs it says "Continuous capture of up to 200 JPEG (Fine/Large), at a frame rate of 10 fps even in 14 bit lossless/compressed RAW files...". This might be read that it can shoot at 10 fps regardless of file format.... But that only JPEGs can buffer up to 200. Many of the announcements we're seeing are based upon this document that might be mis-read. Some other announcements are reporting 200 JPEG/79 RAW buffer (
http://petapixel.com/2016/01/05/nikon-unveils-the-d500-a-crop-sensor-dslr-with-pro-level-specs-and-features/. Even if that's the case, it's quite impressive. In comparison, no slouch either, the 7DII can buffer up to 130 JPEG, 31 RAW, both at 10 fps (slowing very slightly to 9.5 fps for focusing in some cases).
Now, I do use short bursts at times when shooting action that's difficult to time... But mine are usually 3 or 4 frame bursts, occasionally 5 or 6. Almost never any more than that. I already spend more time than I like at my computer editing thousands of images! Sure, I bet someone might occasionally find 200 (or 79) consecutive shots at the 10 fps burst rate handy for something. But by the time I wanted to be shooting like that, I'd probably just switch to video mode instead!
There also is a big difference in shots-per-battery-charge claims: At first glance the D500 offers nearly double (roughly 1200 shots) compared to 7DII(about 650 shots). HOWEVER, a lot of that difference has to do with the 7DII having a built-in flash, which is used for 50% of shots done in the standard CIPA battery test procedure. If the 7DII's built-in flash isn't used, it will get a lot more shots per charge and perform much more similar to the D500. I regularly get 1100-1200+ shots per battery/per charge with my 7D using older, lower capacity batteries. 7DII is said to be a little harder on batteries, hence the new higher capacity LP-E6"N" that Canon introduced along with the model One also probably can conserve even more battery power by turning off the 7DII's built-in GPS, a feature that D500 doesn't have.
The D500 is about 100 grams (20 paper clips) lighter than the 7DII... not sure why this is. However, 7DII has a magnesium body shell, which Canon claims is the reason they don't offer built-in WiFi due to the metal body hindering the WiFi signal. This is purely a guess, but
perhaps part of the reason it's a little lighter is because the D500 uses some plastic panels to accommodate it's built-in WiFi and Bluetooth (as the Canon 70D does). I do know that the D500's top body panel is claimed to be metal (one-piece, in fact, made possible since there's no built-in flash... the lack of which also might save a little weight).
Both cameras have dual memory card slots: 7DII has one for SD and one for Compactflash. D500 instead accommodates SD and XQD memory (haven't been able to determine if both slots can accommodate both types of memory... or if it's one slot for each type).
I shoot stills with my DSLRs. I am not a videographer... so it doesn't mean much to me personally... But I know the D500 is said to be 4K capable, while the 7DII isn't.
Both manufacturers offer deep, extensive systems, though each also offers some unique specialty items that might make one lean toward one or the other. On the whole, Nikkors tend to cost a little more than comparable Canon lenses, and Canon has put more into using fluorite in their telephotos (pioneered development of synthetic fluorite). But both have superb selection of high quality glass, as well as good selection of more affordable options for folks who don't need or want the premium stuff. Some like the Nikon flash system better... Though personally I find the Canon flashes easy to use and quite good too. Nikon FX cameras can be used with DX lenses, with some limitations... OTOH, Canon designs their EF-S lenses so that they won't even physically mount on their full frame cameras.
Canon repair service and support is some of the best in the business... While the same can't be said for Nikon. But today's stuff is so reliable and upgrades so frequent that service may never be needed.
Yes, Nikon has had some stumbles with new product roll-outs. But, to be fair, so has Canon (T6i/T6s sensor spots and 1DIII AF system issues are a couple that come to mind).
In the end, there are more similarities than differences between the 7DII and D500... And in the real world both should work quite well. Anyone deeply invested in lenses and accessories of one brand or the other would take a major hit in the wallet switching systems, so it's great that both manufacturers are now offering top-of-the-line, pro-oriented APS-C models (and one-upping each other). These didn't exist at all few years ago, before Nikon introduced the D200, and especially the D300 models... to which Canon responded with the original 7D, and now the 7DII.