I: Retro but worth a play.
II: Never played.
III: Haven't gotten very far in this (I have the newer ds version) but it seems okay so far
IV: Is amazing, I highly recommend it.
V: Is playable and the job system is nifty, but I thought the story was a bit lacking.
VI: By and far my favorite...
I would definitely say the DS, simply for the much more appealing game library. But I suppose thats a matter of taste, if I were you I'd research and see which had the most games that appealed to you personally and go with that.