The difference is in the type of bearings used, they have the exact info on the speedplay site. It's the difference between cartridge bearings, needle bearings and one more type that currently escapes me. I got the x-2's over the x-3's solely on reviews about reliability, which in turn reflects on the quality of the bearings just before the x-5 was released, I would definately have gotten the x-5 over the x-2, because really I don't think that little weight in the spindle is going to make a difference, and I consider myself a rather serious racer.