Years ago I got a Kokatat dry suit for kayaking down the grand canyon. I could not afford the goretex, so it was full-seal material. When I put it on in my house the day I got it, I was immediately sweaty and thought "no way am I going to be able to stand wearing this on a sunny day. But, in the canyon, it was never a problem. I am not so great a kayaker, so I did a LOT of swimming, in the 50 degree water, and clearly the drysuit was necessary. But on stretches with no rapids, it was seldom too warm, and when it was, I just got wet (by rolling, or by splashing), and I was comfortable again.
So I think worries about overheating in a dry suit are overblown. It will depend more on what you wear under it (I had lightest-weigtht Capilene). And, if you can spend the bucks for Gore Tex, the comfort will be much greater. Bottom line, though, is that immersion in really cold water a dry suit, even with little under it, will save your life like nothing else. I probably will buy another (the first one no longer fits me right
), though I am pretty comfortable sailing in shorty farmer john wetsuit, splash top, and neoprene booties in our "winter" here in southern CA (but the water is pretty cold).
The seals are the inconvenient part. I would recommend getting one with integrated booties, not only because it makes it easy to have warm dry feet, but because it is easier to get out of. I think for sailing I would get a neoprene neck for the same reason. They don't seal as completely, but enough to provide a lot of protection, and easier entrance/exit and comfort wearing. Actually, diving dry suits have neoprene neck seals, that I gather are quite waterproof if correctly worn.