After having two different boat guests this weekend, I see what you mean about the aft cabin. One of them snored so loud I heard him over the large box fan I had set up on the galley counter.Oh, my dear C-34 owner. (and I use to own a C-34 where the main cabin was a great place to sleep with that table down and queen size pit.) But in the C-310 it is not a V-berth, and there is not a skinny end. In fact the end at the bow is about as wide as the end towards the cabin door. (with a wider part in the middle) check out a layout of the 310, you can sort of see the debate. (I liked the C-34 because you had a door that closed off the aft cabin to the rest of the boat, great feature)
But if I were to upgrade, I think I would go for the C350 over a C34. I like the rounder cockpit and the queen center line better then the traditional V birth. Or just really go for it and get the C470.