One theme I liked in X-2 was that of rebuilding/reconstruction, because that process is always filled with uncertainty. In FFX where you break open the truth about Yevon to the people, many like Wakka still cling to their shattered faith. FFX-2 has a New Yevon faction and the opposing Youth League. New Yevon is proof that many in Spira could not have faith in a non-religious world. By contrast, there's that one guy Beclem who's hardcore atheist and harshly admonishes the Besaid Aurochs for doing "ancient meaningless traditions" such as praying. There's also Dona and Barthello. They were tighter than teeth and gums in FFX, but when she joined the Youth League and he joined New Yevon, it wasn't pretty. Interesting that no matter how close you are, where you place your faith can tear even the closest people apart.
Bottom line, the FFX/X-2 saga has some solid thematic elements, but execution was not always consistent.
And, like I said before, I actually agree with Alisha that FFX had my favorite battle system of any FF game.