Yes, I agree.
Riviera: The Promised Land is my PSP favourite game.
Also, 2 Prinny games make an excellent addition.
About RPG's, I think both of them have lots of great titles to choose from.
I bought NDS first because of Castlevania (I purchased it with Nintendogs included, and the Super Card), otherwise I would have bought the PSP (I purchased it with some games: Ultimate Ghosts and Goblins, Final Fantasy 1 and 2, Dracula X Chronicles and Megaman Powered Up).