As you know, I use the 7D. If *all* I shot was portraits in tight environments, I'd have a full-frame and it would be a toss-up for me between the 5DIII and the new 6D with overall cost/value the prime issue. When I'm shooting portraits, I shoot with strobes and on manual. I'd want to compare the AF between the two and see which one I could live with. Another option for me would be a good-condition 1Ds II. They are a great value now and a 1D as a backup would let me use long telephotos with telextenders that bring the minimum F-stop to F8.0 and still retain AF.
I'd keep the 7D for most of my wildlife shooting and for action photography in and out of the ring. I like its AF and love its fps. And its low-light IQ is perfectly adequate.
There is no question that overall IQ is better on a full-frame but with everything, there is a question of what you're willing to compromise. And, what your final products will be.