Disgaea and FFT are complete different forms of the same type of gameplay. FFT is a more serious extremely deep storyline. Disgaea is a good comedy. Disgaea is unique in the lift ability and the random dungeon generator.
The depth of FFT is pretty deep in terms of gameplay. I remember when it first came out and people unlocked the calculator ability, that was pretty deep with a extremely hard setup for understanding how the game mechanics work.
For the GBA, I thought FFTA failed on many levels, it really made everything easier in terms of characters, I was able to win every battle without losing a single guy, and yet annoying as hell with the stupid laws, which they are supposedly getting rid of in FFTA2. I thought Tactics Ogre outshine FFTA by a lot. The TO was fun and engaging. Trying to get all the emblems was pretty fun.
If I recalled FFTA was a Square game as well, and riding on the coat tails of FFT. In fact, to boaster up FFTA2, they made the main character for that game a "secret" character for the PSP FFT.
Never been a big fan of the Shining games or the Fire Emblem games so I can't compare those with these games. If you guys get a chance though, try Dragon Force for the Saturn (been ported to the PS2 as part of a Saturn collection). Highly underrated, and extremely hard game to find.