The camera body will have no noticeable impact on image quality.
Technically the 6D has fractionally improved ISO performance and can focus in slightly less light. But the 6D is an entry-level full-frame camera... it has fewer advanced features. It has a simpler 11 point AF system.
The 5D III has a significantly more advanced focus system with 61 AF points and several advanced features and focusing modes. It's a noticeably more technical camera than the 6D. It also has a better overall build. It has weather sealing. It can hold both a CF card and an SD card.
I wouldn't necessarily say that the 5D III is an action-photography camera per se... as there are a couple of Canon models which are somewhat optimized toward action photography... but the 5D III actually does do rather well in that category considerng it wasn't a primary design feature. It would vastly outperform a 6D in that category. If you were going to shoot action, there'd be no comparison... get a 5D III.
If you're shooting non-action (e.g. landscapes, portraits, etc.) then I don't think you'll find much difference in the capabilities.
The 6D has a built-in GPS... the 5D III uses an external (hot-shoe mounted) GPS (sold as an extra accessory - not included). But the external GPS is much better than the built-in GPS that the 6D uses (it's more accurate, it also has a built-in compass so it records the direction the camera was facing in addition to the location.) One frequent complaint about the GPS on the 6D is that it doesn't switch off when you shut off the power. You either have to pull the battery or you have to go into the menu system to disable the GPS prior to turning the camera off. This was done because it takes several minutes to acquire a position fix when you power up a GPS and Canon feared that someone would briefly switch off the camera (say... to swap lenses, memory cards, etc.) switch it back on again... and then be upset to find out that the next several images have no location info because the GPS didn't have enough time to acquire a position fix. (the new 5D IV has a menu option where you get to choose if the GPS stays on or not.)
The 6D is the "easier" camera to learn and use.
The 5D III is the "more capable" camera -- but that will only be helpful if you take the time to learn how to take advantage of it's capabilities. It's not automatically just going to get better results. But those extra capabilities might make it easier to capture shots that could be more difficult to capture with the 6D.
In terms of the lens comparisons... the 70-200 IS version II is *the* lens to own in the category and pretty much beats everything out there.
BTW, the rumors are very strongly pointing to the announcement of a new 6D which is expected to be shipping within the next few months. The announcement and ultimately the release (when it starts shipping) will put pressure on the price of the original 6D.
And if you really want to save money... go check Canon's online store in the "refurbished" deparment. Generally you cannot distginish refurbished gear from new (other than new cameras come in retail packaging and refurbished cameras come in plain packaging.) They even have the same warranty.
Tim Campbell
5D III, 5D IV, 60Da
