Simply put, I don't think they are designed or intended to be compared to each other. They are intended for two very different types of photographers. I would argue that the 5D3 is at least two, if not three, classes ahead of the T4i. Rebel series, 60D, 7D, 5D3, 1D series.. Although, the differences between the 60D and rebel seem to be shrinking..
(I won't comment on the subjective differences in image quality. Huge can of worms.)
Thanks very much for your thoughts. I am curious though and apologize in advance if it is a can of worms, but what are people's thoughts on the difference in image quality? Right now, there is no other full-frame option and I'm pretty determined to upgrade on that level. But as far as image quality, dynamic range, not so much high ISO ability, but image quality in general- what's the difference between an L-lens on an 4Ti and an L-lens on the Mark III? Any thoughts?
It's hard for me to compare the difference in image quality from a crop sensor body to a full frame body. Once I went to full frame with the classic 5D, I've never looked back, so I really don't know how the rebel cameras perform. I can guess that the "good" crop bodies (ie. 7D) have "good" image quality, and I can say that the 5D bodies have "better" image quality, but by how much? That's entirely subjective. You start getting very hand-wavy, hard-to-quantify answers.
Honestly, the T4i seems to have a great sensor and a great autofocus system (the same 9-cross-type on the 7d?). It looks like a great little camera, so it's entirely possible that the subjective image quality could be comparable to the 7D and any other Canon crop sensor. I think the big questions you have to answer is whether full frame is what you need and whether the build quality is important to you.
You've heard the other benefits of full frame: bokeh, wide angle coverage, etc. Since the "pixels" (or photosites) on the 5D3 sensor are bigger (6.25 micron) than the T3i (4.3 micron, and I assume the T4i has about the same 18MP APS-C sensor), the noise will tend to be better on the 5D3. Sure, the DIGIC 5 will alleviate a lot of that noise, but it will have to work harder on the T4i, giving the 5D3 more opportunity to give you better low light images. Another thing you might be surprised by (I certainly was) is how much brighter and wider the 5D3 viewfinder is. For me, it was a night & day difference when I first looked through my 5D classic's viewfinder. A word of warning, if you decide to go with a combo 5D2+T4i: You might start to feel very cramped by the smaller T4i viewfinder.
If full-frame turns out to be less important to you, I would think the 7D would be a strong consideration, since you want to keep this next camera a while. The materials and construction of the 7D are far better than any plastic rebel, so it will certainly last longer. If you decide you need full frame, you'll automatically get a durable camera, either in the 5D2 or 5D3.