They're just different games. Comparing FFTA to FFT is like comparing FFII to FFI. Sure, you can do it, but the games are so different that it's better to just play the games and decide how you feel about them individually. As far as which one to get, it depends what sort of game you want. FFT is a lot more story driven and has an ability system based on FFV, while FFTA is a lot more battle driven and has an ability system based on FFIX. In both games I got equally psyched about getting new jobs or job abilities but kind of intimidated by how many options you had. FFTA also has the law system, which makes things challenging at first, but when you get further into the game you can actually use it to your advantage to mess up the enemy. FFT is more complex when it comes to damage calculations. The zodiac affinity charts basically required me to constantly look at the manual for help. There's also the brave and faith stats which weren't used in FFTA.

Basically it all comes down to what sort of game you'd rather play.