Go for the one piece. The hump also just kinda gets in the way if you ever wear a backpack or a camelback. Wear what you will wear when you try them on. I went one size up. I wear my one piece until it gets in the low 80's. Above that, I'll switch to leather track pants and a mesh jacket. This is how I dress everytime out.
Things to consider also:
1. I wear bike-shorts and amored shorts under my leathers.
http://www.motorcycle-superstore.com...de-Shorts.aspx
2. I also wear a back and chest protector vest under my leathers.
http://www.motorcycle-superstore.com...c-SP-Vest.aspx
3. They should fit snug without being constrictive.
4. Most are perforated which is great for temps above 65, below that you'll want some extra room for wind/thermal liners.
5. Who cares how people look at you or what they think. Phuk them! Full leathers are the best protection in a crash.