For me, PSP means:

>> awesome graphic
>> good combination of picture viewer, video and music player
>> Internet~WiFi features (Skype, browsing, etc)
>> tons of new and old or remake games of PS2 and such (RPG, strategy, sports, etc)
>> homebrews

Still, NDSL have plenty of exclusive games that PSP do not have and to make things worse, they remake the PSP version of Disgaea BETTER on NDSL... Argh. I would say that the touch-screen feature of NDSL beats PSP upside down as well.

Anyway, people might recommend to get both, but I think either one should be decent enough - I have already find it hard to juggle between all the games on my PSP on a journey, I would faint to imagine myself juggling between two different handhelds on a journey. I was one of those who nearly strangle myself between the two, in the end, I jumped on the PSP bandwagon. Don't regret my decision even for a single second and I will take PSP again if I have to choose again.