I have had my C320 for about 2 years, and have been happy with it. I like that the cockpit has room for several guests, and also like the walk-through to the swim platform. I haven't felt cramped below, but that is a subjective judgment that probably varies for each owner. (I looked at at a 310 when I was shopping, at it was just enought smaller to make a difference to me.) There is a C320 owners association, at www.catalina320.com
, with hull information, etc.
However, the 320 is my first keel boat, and I chose it in part because I thought it was the biggest thing I would feel comfortable singlehanding (which I knew would be lot of my sailing). With more singlehanded sailing under my belt, I think I could have considered the Catalina 36, which does have more room below. If I were doing more cruising I would definitely appreciate the larger head, and larger aft cabin. But if you mostly day-sail (as I do), it may not make a big difference.
Either way you are looking at a good set of choices. Good Luck to you.
C320 Hull #232