they're more alike than different. The new MPA has a little more dense string pattern than the previous graphene MP when strung 16x19. when 16x16 its a little more open. ive seen comments that the XT pro seems bulkier and such, but ive played the current and previous pros side by side and its not enough difference for me to notice. if you compare the unstrung specs that head puts out, weight and balance is the same on the pro, MPA, and S as the previous versions. feel is slightly different, maybe a little more crisp and a little less hollow. maybe.
paint looks better ... it's a matte finish instead of shiny. i wonder if that has an intangible quality that alters peoples perceptions of the two lines.
i honestly think that most people buy the pro just because it says "pro." the MPA and S are much more useable frames for a lot of folks.