People complain that FF1 and 2 on the gba are way too easy: Monsters drop items, mb system, hp goes to 999 (or was it 9999?) etc. Didn't you guys notice that these kind of things are also on FF7-X and nobody ever complained? The gba version were ment for the people who liked the newer FF games and want to try the older games. That's what I think atleast.

FF1 is pretty easy, but the extra dungeons (4 of them) are alot harder.
Also FF2 is harder then FF1 and after you finish FF2, you get and extra story from the characters who have died in the game in the dead world. And this mode is also very hard.

After you finish the games, you also get a music player to listen to all the music that you have heard in the game. And you can always view a monster bestiary if you like.

I think that you should go for the gba version, since they are 2 games in 1 and you can take them on the go.