Advance was simplified a bit and slightly less janky, but the AI in the original FFT is superb. I find the original FFT more interesting on a revisit, at least until you unlock Calculators/Arithmeticians and break the game. It does have one of the more egregious fist mission bumps[1] though.

[1] Tactics games often have a very easy tutorial mission, then drop you off in a first or second mission with an enormous difficulty bump. Sometimes even having a reverse difficulty curve, where the challenge spikes hard in the first mission, then slowly declines over the rest of the game.

My favorite! A2 after it, maybe. But Advance is highly underrated.