In my opinion, Baten Kaitos is a must own, must play for RPG fans who own a cube. The story has some great twists, the artistry in some of the locales is absolutely gorgeous and the music is pretty nice throughout the game too. As long as you don't mind adapting to the card system, I think the game is completely worthwhile for anyone into RPG's on the Cube.
Tales is also good, but it only ends up standing out at all because it fills the gap of RPG on the Cube. It still felt like every other Tales game I've played previously(Destiny, Eternia, etc.) That being said, it was fun and I did complete a romp all the way through it because I liked the story and design of some of the characters(Sheena in particular.)
All in all, I'd say both are a pickup and play for Cube. I'd have to say that BK moreso than Tales just because it offers something a bit fresh as opposed to Tales which while a wonderful game, is just another rehashing of the Tales franchise.