It's all about preference. What are you looking for from a portable? If you like on the go games, quick pick up, no load times, fast and fun, a DS Lite might be the best choice. If you want console games on the go, you'll want a PSP. If you want to also listen to audio and watch videos/movies, you'll want the PSP. And if you want a highly customizable/hackable platform, you'll want the PSP.
Threads of Interest:
The Movie Thread: http://www.vgchartz.com/forum/thread.php?id=6880
The Crow Eating Thread: http://www.vgchartz.com/forum/thread.php?start=0&id=3886
The Betting Thread: http://www.vgchartz.com/forum/thread.php?start=0&id=7104
Custom GIFs Thread: http://www.vgchartz.com/forum/thread.php?id=18963
The Greatest Game Ever Conceived On Any Platform
Tag: "I have tasted Obi-Wan's bitter tears"