Buy NDS and it's successors if you like Pokemon, Mario, Digimon, brain teaser type of game. Some notable hits that are exclusively for NDS are such as Elite Beat Agent, Etrian Odyssey (which I like the most) and so on.

Buy PSP if you like larger screen (still pixelated though) and intend to play retro games as well as emulators (SNES, GB, GBC, GBA, etc).