My response is biased, as I own a P10M, but I looked at a P31-2 and a P31. The P31-2 is a "small" boat as is the P31. By small I mean mostly the cockpits. When I went below a 31-2, it felt very small. I never looked at a 31.
I disagree with TSteele65's take on the P10M being "sparse". I don't know what he means by that. I have had people who have been on ours, who have much large boats, say that the P10M is roomy below and very functional. It can sleep 6 in a pinch, 5 easily. The skeg rudder and deep fin keel help the boat sail close to the weather.
In my opinion, the 10M is a better boat. It is more performance oriented than the 31 or 31-2, yet still has the creature comforts that one would need for cruising. The 31 is a fractional
rig sloop whereas the 10M is a full
rig sloop. The hull speed of the10M is almost 1/2 knot faster than the 31 or 31-2. The hull speed of the 10M is 7.25 knots, pretty good for a 33 ft boat. that has a displacement of nearly 13,000 lbs.
You can easily buy a 10M under $25K. I know several in the New England area for $25K or less.
DrB