Depends on what type of Zelda you like. If you like Zelda games more like Ocarina of Time, buy Twilight Princess. If you like Zelda games more like Wind Waker (But you haven't got it yet, so get
TWW, or at least rent it or borrow it, if you can), get Phantom Hourglass. I also think in terms of using their systems capabilities to their full potential, Phantom Hourglass comes out on top. Nearly the entire game is controlled with the sylus, which is used in really intuitive ways, and works really well. Also the graphics are great for a handheld game. Sure, they're not as good as
TP's, but come on, it's a handheld. However,
TP is longer, has a deeper storyline, and you can turn into a wolf!
But in the end, the choice is yours.