I started with a sleeveless suit but ended up buying a full sleeve suit last year. The water around here is just too cold for me to wear the sleeveless until around mid August. It is a little more constricting, but you get used to it, and it gives me a few extra months in the water.

You'll have to decide based on how cold your water is, how much you need the extra buoyancy, and the price. Xterra tends to frequently have half priced sales on most of their suits, so if you want the full sleeved just wait a little bit and you'll be able to get it cheaply, too.