Quite honestly, I enjoyed Tactics Advance more. Now, before anyone jumps on me, I've got a few reasons:
1) I didn't get far in the original Tactics when I tried to play it this summer. It just felt too slow and I wasn't willing to devote the amount of time to it. Maybe TA was just as slow and I was just in a different mindset at the time.
2) I didn't really care about TA's story, I just loved the way it played. I really like the gaining abilities from weapons system, as well.