I have sailed with both the one piece and the 3 piece tramp.
When I just sailed out on the lake for fun, the one piece is decent, but does have some draw backs that I dont care for. One being that if you dont really have a place to lock your feet against when you are not trapeezing out on the wire, but just hiking a little bit. I know that there are straps there for that, but I haven't really used them.
Yes the pocket is nice for putting things in while you are sailing, but you can also sew in more pockets for that if you need them.
I am going to switch back to the 3 piece tramp in the near future so I can be class legal and get my boat tighter for sailing.