I'm thinking to change my wheelset to Shimano XT hubs and Mavic rim. Here is my question: Which Mavic (717, 517, 221, ...) I should use? I mainly do Cross-Country and All-Mountain, very few Free-Ride, extremely few Down-Hill. BTW, what is the difference between Shimano XT and LX, besides the spellings.
I personally don't favor Mavic. Check out some DT Swiss or Velocity rims. It is also worth it to go out to your LBS and see if they have a skilled wheelbuilder. A good wheelbuilder will be able to customize a wheelset to fit exactly your needs if he knows your info (weight, riding style, craziness, etc. can all be used to customize spoke patterns and specific hubs or rims).
If you don't have a good wheelbuilder locally, I'd suggest giving my LBS a call. They are the best around, hands down. 888 577 8837.Ask for Ben. He should be able to help ya out with a custom handbuilt wheelset by him.