The problem with asking the "Nikon or Canon" question on a forum like this is that you'll get the Nikon fanboyz and fangirlz recommending their fav... and the Canon shooters like me will recommend their brand. I'm a little surprised no one has spoken up yet for Sony or Pentax or Oly... That will probably happen soon, too.
Very few people here will have much hands-on experience with multiple brands. (I happen to have and use some Nikon, Pentax, Oly and others... But they're all vintage cameras in my collection. My modern "daily user" DSLR system for work is Canon gear.)
Really, all brands offer very capable, entry-level models like the Nikon and Canon you asked about. Any of them can be learned to use well and take great shots... It's mostly just nuances and small conveniences that set them apart... Such as the lack of a self-cleaning sensor on the D3400 that some people have noted. BTW, actually the Canon T6 doesn't have a self-cleaning sensor either. I believe the similar, but smaller Canon SL1/100D
does have one... As do the T6i/750D and T6s/760D and all other current Canon DSLRs. A self-cleaning sensor is a nice feature I personally really wouldn't want to be without! I often find myself shooting in dusty conditions. Even though I use multiple cameras to minimize lens changes and risk of dust getting inside, years ago when using cameras without self-cleaning sensors I had to do manual cleanings fairly frequently... every 1 to 3 months. Now with self-cleaning sensors it's necessary a lot less often... maybe every year or two, with reasonable care.
Whatever brand and model you decide to buy, you can probably get more bang for your $ buying a "kit" that includes one or two lenses. For example, the Canon EF-S 18-55mm IS STM is usually included with their entry level models and is a decent lens, though a bit plasticky (all lenses will be, at this price level). If you get a two-lens kit, the second lens will nearly always be a telephoto zoom. With entry-level models they often come bundled with either EF-S 55-250mm IS STM or EF 75-300mm lens. AVOID the 75-300mm! It's simply not one of Canon's best efforts. The EF-S 55-250mm IS STM has much better image quality, is closer focusing, has faster/quieter STM focus drive, and has helpful image stabilization. The 75-300mm falls short in all these respects.
Some of the biggest retailers let you design your own "kit". But beware of "bundles" from fly-by-night outfits online. They often substitute lower quality items that really don't add up. Stick with the major, long-established, reputable dealers, to be safe. There are lots of shysters online!
You also should look beyond the basic camera (and kit lens, if you buy that way). What other lenses and accessories that you might eventually want are offered? How good are they and what do they cost? For example, if you also want a wide angle lens Canon has one of the best deals of all: their EF-S 10-18mm IS STM is the smallest, lightest, least expensive by far, and currently the only ultrawide with image stabilization (which really isn't all that necessary on an ultrawide, but is nice to have since it can do no harm and doesn't appear to add cost). The EF-S 10-18mm costs under $300. The most comparable Nikkor AF-S 10-24mm DX costs 3X as much! Almost $900!!! In some other comparisons, you may find that Nikon or another brand has some advantage. My point, though, is to do some comparison beyond the camera itself.
Or, do you want to be able to use vintage gear... such as old manual focus lenses? Many old Nikkors can be used on modern Nikon DSLRs. A lot of old Pentax lenses will work on Pentax DSLRs and vintage Olympus often can be used on modern Oly cameras. Sony (Minolta, Konica-Minolta) and Canon actually cannot use a lot of their own more vintage lenses. However, via simple adapters both Canon and Sony can accommodate a lot of vintage Nikon, Pentax, Oly and other manufacturers' old lenses. In most cases they will be manual focus and aperture control only, of course... but there are literally millions of excellent vintage lenses that might be bought at some savings and can be a lot of fun to use.
Also consider third party products offered that are compatible with whatever system you consider. For example, most third party lens manufacturers make their products to fit Nikon and Canon. A bit less, but many offer Sony compatible items, too. Somewhat less things are offered for Pentax, Olympus and some other brands of cameras. In addition to lenses, you might also want to compare other accessories such as flash, battery grips, etc.
Look for some online reviews and tests of the cameras you're considering. There's a whole lot of info out there about specific models. Professional reviewers have more hands-on experience with a variety of gear and may give you more useful insight than forum participants or user comments on retailer websites. Bryan Carnathian, Tony Northrup, Michael Reichmann... heck, even Ken Rockwell... dpreview.com, techradar.com, imaging-resource.com, cnet.com and many other people and websites offer extensive reviews. Google is your friend (searches for "Canon T6 review" and "Nikon D3400 review" both found over 400,000 results!)
Canon USA has had an online store for several years, where they sell refurbished cameras, lenses and accessories at some savings. That can be a good deal, since their refurbs are often hard to tell from and have the same warranty as new. I think Nikon has recently started doing similar direct sales.
Do be careful, though. I was in the market for two cameras last year and was about to buy them refurbished, when I discovered that between sale prices and bundled items I could actually buy brand new even cheaper. There are various promotions from time to time throughout the year, that might make brand new a better value than used or refurbished!
Have fun shopping!