Well, I own both a 7D & 5D Mark III, and I've shot the same subjects using the identical lens on both cameras. I bought the 5D, in part, because it is nearly identical to the 7D ergonomically, but primarily to obtain better low-light performance & full-frame. So, in my experience, the 5D locks on autofocus faster and more consistently than the 7D. Noise, even at iso 100 is much better, which is very obvious at 100% viewing of, for example, blue sky areas. As for low light performance, I'm in complete awe of the 5D, with it's iso 12800 roughly matching my 7D's iso 1600 performance, visually. As for dynamic range, I use a Sekonic L-758 light meter for some of my work, which can be calibrated to match the dynamic range of each camera. Using Sekonic's targets & calibration software, the 5D gives me about 1.5 stops more range than the 7D, too. I'll also say that I LOVE, really LOVE the silent shutter mode on the 5D, which is really nice for candid shots. So I'm one happy girl, and the 5D is worth every penny I paid, at least to me. I still have the 7D, and I still like it, but I was really surprised at how much better the 5D is.
Anyway, I doubt you would go wrong either way, but I do VERY much like my 5D & 7D cameras.