i think u should get nds. as we know, not many good games for psp unless u wanna play old ps1 games. ps1 got plenty of great rpg, final fantasy series, xenogear, star ocean, etc...
-psp is okay, u can enjoy games like riviera, gurumin, jeanne d arc and monster hunter. other than rpg, these title can be consider as a good games:-
metal gear solid portable ops, killzone, ultimate block party, megaman powered up, etc...

nds got many great games. seriously i can't list down all the good titles...

good choice.