Upload & Sell: Off
I own the V600 and have used the V750 extensively. The V700/50 can do 4x5 for one thing, so that's a big difference. You can do contact sheets with a 700, whereas you can't with the 600.
I've been able to do okay scans of 120 film with my V600. I think the V700/750 holders are a bit better, but the 600's holder is okay. The 700/750 lets you do two strips at a time, whereas the 600 limits you to one strip (which is essentially two images, at least if you're doing 6x7 like me).
The software's a bit different, I think, but not much. It's mostly just a different download.