I've not played Skate It, so I can't comment on that.
Multiplayer is one board only, regardless of how many boards you have.
We Ski is a very mild, family-friendly game. Shaun White utilizes the board a lot more, allowing you to do tricks, speed up, slow down, and so on. We Ski only uses the board for turning; tricks and speed use your remote/nunchuck combination. It's cute, and it makes the game more realistic, but it is no Shaun White, hehe.
From what I've heard, Skate It is a more technical game; most of the tricks in Skate It are difficult to pull off / master like in reality. I think Shaun White focuses more on arcade-style action, or it at least focuses more on arcade-style gameplay than Skate It.