I used the long one from Underworks (was in 2001, forgot, but it was the one that goes down to the waist,) and still use the TriTop, it was either small or x-small. I'm five foot two and under 100 pounds.
The Tritop is perfect for me, doesn't roll up, doesn't cut into my ribs, the arm and shoulder areas are comfortable, and I can even wear tight and thin shirts and no lines show. So I have very positive feelings about the Tritop.
The long binder I had for a short while before I lost it, was nice, but I had to keep it pulled down low for it to even bind, and if I had to go a size smaller I would not be able to breathe in it, much less step into it. That's all I can tell you, besides researching LJ groups (ftm has WONDERFUL keywords about binding and people give their experiences there.) Have you checked that yet? Was how I made my purchase decision.
Good luck on this.