I have a psp and a ds lite.
THe PSP is way better. Typing with the psp goes alot faster than on the ds. Why? I am used to cellphone writing (send textmesaging, etc). And on the psp its similair. Also PSP is way faster than the ds and the resolution is bigger, wich makes it comfortable to view webpages.
O and right now, I'm typing this message from my PSP :)