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.