The PS1 origins of FFI and II and the DoS GBA version are just about the same, the only difference is the added features. The GBA version has an extra play mode once you beat through FFII, and the FFI version has 4 added dungeons and many added bosses within those dungeons.

Whilst the PS1 version features some of the original artwork of each game, and also a really neat cut scene in the beginning for FFI. Not very lengthy or all to big, but pretty neat none the less.

I think I just about covered the difference in the FFI and II versions.

As for the others. FFIII as of yet is not released outside the original NES Japanese version. So your best bet is to just either get a ROM of it somewhere (the EoFF main site has it I do believe) or wait for the remake to come out, but I'm not sure of when it will be.

FFIV you can get in FF Chronicles, and FFV and VI are both featured in FF Anthrologies, both Chronicles and Anthrologies are PS1.

In my opinion though as to what to get, I would say PS all the way. Although you would have to deal with some rather slower then average load times, but eventually you just ignore them, while the GBA versions don'twon't have any loading time issues. But I find the extra features on the PS versions to be a bit nicer on some of the games. (In particular I enjoyed the artwork of FFII on the PS1 version)

Well I hope this helps you in reaching a decision.