Planet Puzzle League by far. I own both games and really got my money out of Puzzle Quest, but after you reach level 20 or so the game becomes far to easy. The amount of things your character can do get quite obscene. PPL single player experience isn't as robust, but the multiplayer rocks. PQ is great, but has little replay value outside of making another character. PPL has that tetris like longevity, especially if you have some people who will play Wi-Fi with you (me, me, me!!).
So, the long and short of it is: If your only going to get one, choose PPL. If your going to buy both, get PQ first and the PPL. Or follow Lord's advice and hit Toys R Us this weekend and get them both and a copy of Advance Wars for free!