RV6 is a little harder to build
I bought one of the 6A quickbuild kits that became available after the 7 came out and a bunch of people changed their orders. If you've found a QB 6 that hasn't been started, that's quite a find. Personally (and I'm obviously biased), I've always thought the 6 had better proportions that the 7. It just looks sportier, especially with the original rudder. And I really like the full carry-through laminated spar.
By the way, it's easy to tell which rudder you've got. The bigger rudder has a balance weight.
The biggest drawback to the 6 is that it's quite a bit more difficult to build. The plans and instructions are downright lousy compared to the later kits. For instance, when you get to the firewall-forward section, there's a whole chapter on how to choose an engine -- but then it refers you to Tony Bingalis's book for instructions on how to actually layout the firewall and install engine. I wound up buying a 7 FWF kit and throwing half of it away mostly just to get a plan to work from.
The great thing about a QB is the quality of workmanship. Man, those Philipinoes do good work! There's not much you can do to screw it up (and I sure tried!). Mne flew straight -- ball centered and feet on the floor -- from the first flight. I never needed any trim tabs.