I wear a one-piece Zoot trisuit. It has the following advantages:

1. More aero
2. Has a slimming effect on my spare tire
3. Easier to deal with on the swim
4. More comfortable on crotch (in the same way bibs are more comfy on a road bike)
5. I can roll down the entire top, effectively becoming shirtless, without actually removing the top

There are a few disadvantages that come with a one-piece, though:

1. Harder to pee/poop, especially when wearing clothes *on top* of the suit (aka cold weather)
2. Usually more expensive than a tri top + tri bottom
3. Fewer options/color choices
4. May be harder to fit on some people (not true for me personally, mine fits fine)