The 6D Mark II is 26MP and has the same AF 45-point system as the 80D, 77D and T7i. While that AF system is a HUGE improvement over the 11-point system of the original 6D (20MP), some complain that the array of AF points is somewhat centered in the full frame camera's image area (almost as if Canon didn't scale it up for the larger format, but simply used it "as is" from the 80D, etc.) The 6DII's AF system is all higher performance "cross type" points. The original 6D has only one cross type point (at the center), the other 10 are lower performance "single axis" type. Both cameras are able to focus as low as -3EV (with limited AF points, only one in the original 6D). The 6DII is "f/8 capable" at up to 27 of its AF points (varies depending upon lens). The original 6D is "f/5.6 limited". What this means is that you can use teleconverters with more lenses on 6DII. For example, on the newer camera you can use a 2X with an f/4 lens or an f/5.6 lens with a 1.4X.... neither of those combinations would be able to autofocus on the original model.
The 6DII also has a slightly larger and higher resolution LCD screen, which is articulated and a Touchscreen (the original 6D's is not articulated or a Touchscreen). 6DII also has Canon's Dual Pixel Phase Detection AF in Live View and video mode. This is MUCH faster and more useful than the contrast detection AF that the original 6D uses.
6DII also has "Anti-Flicker" feature (which was introduced on 7D Mark II and is now on almost all Canon DSLRs and mirrorless cameras... earlier 6D lacks this feature). When it's enabled, Anti-Flicker detects the cycling of fluorescent and similar types of lighting, timing the release of the shutter to coincide with the peak output of the lamps and making for MUCH better exposure accuracy under those types of lights. Without this feature, it wasn't uncommon to have half or more of your images under fluorescent lights to come out badly underexposed. With Anti-Flicker the number of underexposed images is reduced to almost zero.
There are other differences, but I think those are some of the primary ones. For a detailed review and to compare with any camera, I'd recommend Bryan Carnathan's
www.the-digital-picture.com His reviews are very fair and helpful. There are also image quality examples and other tools for comparisons, if you wish to use them.
42nd Street IS NOT a good place to buy. You will find many complaints about them online, if you do a search. The 6DII is on sale for $1500 at reputable stores (Adorama, B&H, Amazon Direct, etc.) Anyone offering one for $1200 is almost certainly going to pull a "bait n switch" on you. Order from them and you'll get a call the next day telling you the battery and charger aren't included, offering to sell them to you for $200... They'll probably also tell you it's the "Chinese" model. If you want the menus in English, that model costs $200 more. Etc. It's all utter BS, but if you actually go along with it all and buy from them, you'll almost certainly end up paying MORE than you would have if you'd bought from a reputable store. Oh, and 42nd Street's is probably also an "international" model, aka "gray market", without a US warranty. Anything happens, repairs will be at your expense. They might offer a "store warranty", but good luck collecting on that if anything happens.