I am not a 20 year photography vet or anything, but I have highly analyzed Canon's line and I can tell you this:
Full Frame bodies are in the midst of a pretty significant transition that probably will be mostly complete by around 2015.
Let's look at a couple of areas:
* Price. The 6D was one of the first "cheap" full frame cameras. Will it be the last? No. As one of the first cheap full frame cameras, there is a lot missing from the 5DMKIII that would be extremely useful - such as the autofocus system mentioned earlier. Or, even for longevity purposes the weather sealing and some of the controls the 5DMKIII has.
* However, even the 5DMKIII has some key features missing that Canon is currently beta testing with the APS-C users. Examples:
-Touch screen: Yeah, I know it is not 100% needed, but it allows you to INSTANTLY pinch zoom to EXACTLY where you want in image preview to see if you got a shot focused just as you wanted. Scrolling through images is also more intuitive, and menus can be made more intuitive via touch as well. And if you do any video at all, without touch you lose the useful touch-to-focus system.
-Wifi: Nice for both remote trigger and could be used at some point for high speed realtime backup. You don't need someone running SD cards to back 'em up if you have a laptop with an 802.11N/AC wireless router that the camera is streaming pics to for backup in the background. Now the 6D does have WiFi (as does the 70D), but it is far from reaching its potential in the field.
-"L" lenses missing IS: Image Stabilization is useful for video, but it is also useful for camera shake, especially in low light/long focal length. But, again here we are seeing a slow uptake as it is tested in the less expensive lenses (with some exceptions of course in the 100mm and over range), refined, and likely will eventually end up in the "L" lenses over time. It is odd that there is an EF-S 17-55mm f2.8 IS, but there is literally no similar focal length zoom of equal speed aperature w/ IS in the Canon EF lens lineup. Likewise we are seeing the introduction of consumer primes with IS across the board while this has not really happened in L lenses yet except in some of the lenses 100mm and longer; you might say IS is not needed, but it is nice to have the option of using. I would bet in a couple of years much more the the L-lenses will offer it.
My strategy: So in summary, the 6D and even the 5DMKIII are missing some key features that it is very likely their successors will have - as things like touch and wifi mature & become an expected feature - while at the same time the price of entry will likely go down as well.
While waiting for that to occur, I would hold on to your T3i until next spring and see what the 7D MKII brings. From the rumors, it looks like it might have some of these features (touch, wifi, excellent autofocus) as well as a refined sensor that may deliver less noise than the 70D. You could also see things added like a headphone jack for audio monitoring that would be immensely useful if you shoot video. In other words, it would be a nice fully featured upgrade to make while the full frame cameras continue to evolve and drop in price...
I've always heard the mantra that a wedding photographer needs at least one set of backup gear, including lenses. It might be interesting to have a setup that is similar to your subject: an APS body, an APS-C body, and a set of lenses that go with each with perhaps some for both (like the 70-200IIL may be too costly to duplicate, but you could get something like a 70-300L for instance to supplement it). Maybe keep your 24-70 for the APS, but buy an EF-s 17-55mm IS for the APS-C (the latter's FOV might be a little more workable with the 1.6x crop). Or, keep the 70-200ii for both, and pop it on the APS-C when you want to make use of the 1.6x reach. I would think it would be nicest to have both APS and APS-C because with Canon's implementations they are both strong in different categories (APS: detail/noise, APS-C: reach/speed).
While it is true you could spend forever waiting for the latest and greatest, I really do think touch, wifi, and the newer autofocus systems (if you were thinking 6D) are worth waiting for. They are not only nice to have, but can significantly improve the end result through quicker target acquisition through improved AF, much faster QA through touch zoom, plus improved backup potential through wifi. And they will most likely be available in all the full frame bodies in 2014-2015, but APS-C has 'em now.
