I agree with Lise. I've used a sleeveless wetsuit in all my tris so far. I've only had one where the water was pretty chilly (about 67 degrees)- but once I got going I never noticed it again. There is usually a big difference in price between a sleeveless and full sleeves, so if you think you'll be doing most of your tris in the late spring through early fall, a sleeveless should probably be ok (if the lakes you're swimming in are not too cold). You're in NY tho, and water temps can be considerably cooler than in the midwest where I live.
A REALLY nice long sleeve suit will be built where the shoulders will have less material and more range of motion, but will be thicker (more bouyant) in other key areas. I certainly know that a full suit will be more bouyant overall, but really, in my case I just really need the extra bouyancy in my gigantisaurus butt/lower half where my sleeveless suit covers anyway.

Can you ask other trialthletes in your area for opinions on what they generally wear? Others in your area may have better experience with lake temps/ comfort in the two kinds.

Best of luck to you!!!