Gaming wise. tie.
NDS has alot more game choices over PSP.
However, PSP games are.. 1 word "solid."
Price wise. nds wins.
NDS is considerably cheaper than PSP.
Interface wise. psp wins.
A PSP looks alot more expensive than it actually costs

NDS looks like the perfect gaming console.. for ages 12 and below. And with its dual screen. Unless you have eyes of a chameleon, It may prove difficult to focus on two screens at a time.
Other features.
PSP is able to go wireless. View MP4 files and plays wma/mp3.
NDS is not able to go wireless and you would have to purchase additional wares to enable your NDS to view movies and play music.
My verdict.
Unless you are really on a tight budget and is a hardcore pokemon fan.
Get a PSP.