It will be warmer in Feb, but if you are planinng a trip accross the salt flats it will be really wet there and you probably won't be able to drive across it like you can in the dry season. It's still REALLY beautiful when it's wet though, although travel on local transport might be a bit more difficult with roads too wet to travel on in some areas. (Tupiza to Uyuni mainly)
If you go in the dry season, it's incredibly cold on the altiplano so both times have their pro's and con's. It's definately less busy in the wet season.