I liked the alternating/selectable job system in X-2, in general, better than the 'switch out characters to make sure everyone gets XP' in X. Plus the sphere grid seemed cool at first, but then seemed somewhat linear and limited; the job/garment grids allowed a lot more flexibility. And I liked the 'chapter/mission' format, easier to play for brief periods of time.
Anyway, they're both great games, and 16.88 is an excellent price.