In my opinion, they aren't comparable. Both systems have their own games, own kind of graphics and own kind of way to play those games. I have no constructive criticism on this though
it's how I feel about the two consoles. I'm a kind of graphics-whore so I go for the PSP (I got an NDS lite and PSP myself). I've really been enjoying the NDS before I had a psp. It's just that you can customize it with a nic background, put stuff on your memory card (I'm not into the psp hacking thing), the screen is crystal clear.
If you use it properly, the screen.. no the whole psp will stay pretty and shiny.
People complain about the battery life and the screen brightness, well they're good enough for me. It's a fair trade.. cool (3d) graphics for a little less playtime. I'll just plug in my adapter and it's charged again.
Plus I think it's amazing for such a little machine to have such pretty looking games, and best of all are the Japanese rpg's that come out for it.