It seems that FFTA, is either loved or absolutely hated.
I personally HATED FFTA. I thought it was a horrible game. The only ties that FFTA and FFT had was the land itself and the name Ivalice. It was far too easy for me, but that was only because by the time I had beaten FFTA once, I beat FFT twice beforehand. FFTA's Quest system killed it for me, some of them were just far too repetitive, involved just... killing enemies. FFT actually HAD an intriguing storyline and I loved the characters, especially the Beowulf+Reis sidequest, and the classes were very unique, moreso than FFTA. I didn't find it very complicated at all, but that's just me. The diversity and replay value found in FFT is great, and there are so many characters to get to know. I found FFTA's plot a bit strange, but that's also just me...
I'd suggest that you get FFT if you don't mind the length and the learning curve. But FFT still beats out FFTA in many aspects.
Though by the looks of it the next 50% of the posters will begin flaming me. ^^;;