You must have gotten the deal of a lifetime on your ST for that price.
Few differences:
X3M - No third row, but is only a few hundred pounds lighter than an ST. Non-comp version is 70K, not much more than the ST new for new, and runs low 12's, high 11's stock.
X5M - Weighs over 800 pounds more than the ST, the non-comp version is 40K more than the ST, and is a true luxury car with air suspension and other features.
Tune only, both the X3M and X5M make over 600HP.
The M Package versions also run pretty quick and are much more affordable.