I actually made the same moves as you John, and for the same reasons. However, after using my 6D for only a few months I soon realized that I sorely missed the more advanced AF system of the 5DIII and have since sold the 6D and bought a 5DIII...again. Although the 6D is truly a superb FF DSLR, I found myself (as an amateur) unable to get a sufficient keeper rate using fast glass and the outer AF points, therefore I would end up using only the center AF point and cropping in because focus-and-recompose at < f/1.4 is not especially effective. Although I do miss the aforementioned features of the 6D, including the -3EV center point, built-in Wi-Fi and that all of the buttons are located on the right side for easy, one-handed access, the 5DIII is in fact a better camera and is therefore priced accordingly.
Overall, I did lose a couple hundred bucks by selling the 5DIII to pick up a 6D, and then selling the 6D to pick up a 5DIII, so if I can give any suggestions from my experience it would be to first and foremost go with the one you can afford and if that is either of them, then go with the 5DIII. The only reason I can see someone downgrading from a 5DIII to a 6D is for cost reasons (as is the case with the OP and me), whereas many 6D users perceive the 5DIII as the next step in the upgrade path due to the more professional features (AF system, dual cards, robust body, etc.), hence the cost difference. Either way, both are excellent bodies and offer some noteworthy features over one another, but if both cameras were priced the same I'd be willing to bet that nearly all of those saying that the 6D is a better camera would end up purchasing the 5DIII or eventually exchanging for it because overall, it is a better camera (although from a $/performance standpoint the 6D clearly wins). Just thought I'd share my experiences
What's the light condition looks like at -3EV? Who photograph at -3EV?
6D might have the advantage with -3EV(on paper speaking), however, will it focus accurate under that light condition or even under -2EV?
To me, having 41 cross-type AF points and 5 dual diagonal AF points are MUST better than 1 AF point camera.
OP will miss his 5D III when he needs to shoot something like this: http://albums.phanfare.com/isolated/t302HPVC/1/6306064
Interestingly, one of my primary uses for that is when using a heavy ND filter (10 stop). Traditional focusing is almost impossible (viewfinder is dark and even live view only shows so much). When I used a 5DII I often would have to mount the ND filter after focusing, running the risk of slightly affecting focus. The 6D can focus just fine with my ND filter attached.
My 5D III focus just fine with 10stop ND @ f9 TWI by Dustin Abbott
My 2cents: There is nothing that 6D can do that 5D III can't - except wifi + GPS feature. Same thing to 1D x, there is nothing 5D III can do that 1D X can't - except better.
I do look forward to be an owner of 1D someday and I'm sure we all look forward to that day - to shoot with better and faster camera
Here are some photos taken with my 5D III at my house today. California time 9:30ish PM. All lights were off. The only light source was from single wood log, burning in the fireplace.
1. First photo - just to show you the lighting around the fireplace - almost no light. There is a Christmas tree on the right hand side of fireplace. The tree is about 5-6ft away from the fireplace. - NO PROBLEM FOCUS ON THE FIREPLACE GRILL/DOOR
2. Second photo - taken with 5D III @ 12800ISO - NO ISSUE FOCUS ON THE ORNAMENT
3. 3rd photo - taken with 5D III @ much slower shutter speed - NO ISSUE FOCUS ON THE ORNAMENT
Again, I don't understand WHY
people complaining about 5D III can't focus in the dark.