The only diff in the seat base is the lugs that are on the XC base to hold the air box lid down are not on the SX base.
The shape, size and all else on the SX seat plastic is identical to the XC.
As bradmulvey said, it's only the cut on the foam that is lower on the SX.
Tested one on my XC yesterday. Tried to take off with it, but the owner might have noticed