Ahhh The DS, what once was my faveourite handheld ever, that is untill I got a PSP (sold my DS to get it), call me shallow or whatever but I prefer a system with power, which is something the PSP has over the DS , I also like it for its features (web, video, music etc.) another thing it has over the DS, however there are some things, the DS does that the PSP dreams about, such as, using the tough screen to controll an FPS like in Metroid Prime Hunters (exellent game) which easily outshines the PSP's Triangle to look up X to look down etc. used in coded arms, which turned out terrible despite the visually stunning effects and such.
But any way to answer your question, I think the NDS lite is worth it, and you should get it.
Although if I was you, I would keep the PSP because IMO the PSP is better, but I'm not you, so thats up to you.