One is a classic JRPG and one is a classic Western RPG. JRPG has a turned based combat system that is generally very menu driven. The western is going to be more Icons and point and click. Final Fantasy being the J(Japanese) RPG
Baldur's Gate has more detailed stats and is more action oriented, also 10 years more recent so probably more accessible to most people.
But 3 has had more re-releases so BG being most recent not so much.but why are we comparing two uncomparable games?
You are preaching to the choir. If the world forced me to chose between FF3 and Baldur's Gate then it's be a pretty good situation, you can't go wrong either way. Get them both. Is it bad that I've considered getting an iPad just to enjoy Baldur's Gate again? FF3 is having a time limited sale, Baldur's Gate will likely not increase in price so if finances are the only concern I say go FF3 and get Baldur's Gate as soon as possible later.