One pieces by their nature are much warmer than two piece combos, just like a mitten is naturally warmer than a glove. So that's a definite plus for them. Also, it means fewer things to get them into when it's time to go skiing. However, there are drawbacks to the one piece. The biggest one is having them try to struggle out of it when it's time to use the bathroom (especially when its time to do #2). Also, the one piece is great for when you're outside, because it keeps you toasty warm. However, that becomes a drawback when you're inside, for lunch, breaks, at ski school, etc. Keeping the whole thing on is far too hot, and your kids will overheat. Taking the whole thing off is a huge pain, because you'll have to take the boots off, and your child better have on decent underlayers. So what ends up happening is you pull the top half off, and tie up the arms to try to keep the bottoms on. Which usually doesn't work, and your child is either holding them up, or just letting them fall down.
Personally, one pieces drive me crazy, especially when parents drop their kids off the instant the school desk opens, because that means almost 2 hours that first timer kid is going to be inside, and the last half hour inside we expect him to be doing other things with his hands other than holding up his pants.