Zeros are newer than the X's. Personally, I don't see any point to the X's anymore except I believe that they are cheaper. The Zeros are adjustable and have a better cleat spring design that doesn't get fouled as easily.

Merlin, I don't believe that the LA's are much different as far as clipping out but they are much easier to get into. Also it was pointed out here a little while ago that LA's and Zeros are interchangeable, that the difference is in the cleat. I tried it with my wife's pedals and my zero cleats and it did work.