For the most thing MPs are exactly the same as XPs, only MPs can run in dual configuration and positioned toward workstation/server market, while XP is for single CPU desktop systems..
They have same ammount a cache, but MPs could be slightly more reliable because, I think, they are/were tested more rigorously.
sgtBaker: you're thinking of XP-Ms or XP Mobiles: these have lower vcores and unlocked multipliers, and can overclock better than their desktop counterparts.