One important note to remember is that "full frame" camera bodies cannot use "EF-S" lenses. Your T7i likely came with at least one EF-S lens and you may have more. That would meant that upgrading your system could involve not just the body purchase, but also at least one lens purchase. The upside is that once you've upgraded, those lenses work on all EOS cameras and Canon's best glass is made for these full-frame cameras.
Canon's Rebel series bodies previously had a very simple focusing system (9 AF points ... a center point and 8 addtional points arranged in a diamond pattern). Meanwhile their pro cameras had advanced focus systems.
Those advanced focus systems finally made their way down into the Rebel line and the T7i has a focus system that's somewhat similar to what a 7D had a few years ago (not quite the same, but similar).
So while the "Rebel" series are generally priced and meant to be consumer-friendly entry-priced cameras... the T7i is the most significant upgrade to any Rebel series body in many years. Most previous new cameras offered incremental improvements over the prior model... the T7i offered quite significant improvements over previous models. It finally hits a point where it's blurring the line between the entry level bodies vs. the mid-range bodies. It does this so well... that it's sibling, the 77D (which has identical sensor and specs... but adds an extra control dial and an extra LCD screen for easier use) actually is classified as a mid-range body. The T7i is no slouch.
The 6D, meanwhile, still has a focus system which more resembles the system in previous rebel models except it has 11 AF points (it looks a lot like the 9 point system except they added an extra focus point just left & right of the center point.) Also the 6D's continuous burst speed is about 4.5 frames per second. The 6D was introduced as an "entry level" full-frame body. Previous full-frame bodies were always about $2500 and up. This was the first full-frame body in the industry that was introduced at a price point less than $2k.
The 6D is not optimized for action photography. It is, however, very good at low light situations.
The 5D III has a very advanced focus system that can better handle action photography. It doesn't have the blazinglly fast continuous burst speeds of the 1D X bodies (up to 16 fps for the 1D X Mk II with a buffer large enough to hold 170 RAW files) or even the 7D bodies (10 frames per sec for the 7D Mk II with a buffer large enough to hold 31 RAW files). The 5D III is about 6 frames per second (the 5D IV is 7 fps).
Your T7i is also rated for about 6 frames per second.
You may find that your T7i is a bit better at the track shooting action photography...
But you will also likely find that a 6D would be better at portraits, landscape, and low-light.
The 5D III would pull ahead as it has a better focus system than the T7i, a bigger buffer, and the same 6 FPS burst speed (it's harder to get a fast burst speed when the mirror and shutter are physically larger). Basically it would be an upgrade all the way around... but will cost a bit more than the 6D.
A refurb 6D on the Canon online store is about $1200
A refurb 5D III on the Canon online store is about $2100
(note these are in the "refurbished" section. A "new" copy will cost more.)
...and of course these prices don't include lenses (in case you have to upgrade a few of those).
Tim Campbell
5D III, 5D IV, 60Da
