While I prefer N+ for the PSP, the DS version is absolutely awesome. One warning though - the "online" play the DS game touts is actually not functioning and hasn't been all year long; it worked for about three months after release. Then Silverbirch went out of business and Atari was supposed to handle the servers. I've contacted Atari several times asking them when the servers for the DS and PSP versions of N+ were going to go online and they said "in a couple of weeks." Well, months passed... and then I commented on Metanet Software's blog about the situation and they told me Atari was not going to handle doing the servers for both copies of the game.
That being said, the single player experience is absolutely fantastic. And I definitely recommend N+ at that price.