I have 9 PSP games, 5 DS games.
It comes down to this. Owning both is great. DS is fairly priced so its possible, for one.
Its just DS is in a situation where there are plenty of good games, but finding them is hard since distribution is ass. If you are going to get DS or HAVE a DS, pre-order always.
PSP has plenty of good games too, but you can FIND them, I own a lot of great PSP games oh shit cats attacking my hands...anyway, people who say PSP has no good games are clearly fucking stupid.
People who say the DS is just a bunch of ports and rehash are right..almost everythin NOT from Nintendo is, and most of Nintendos stuff are sequels and 'ports' but its good ports ua777888damn fcat. I dunno, I mostly use my DS for GBA games anyway so whatever.
In short, own both if you can, you really cant go wrong. Games I recommend for PSP:
Wipeout Pure - 20 bux now
Hot Shots Golf Open Tee - 20 bux too!
Daxter - Masterful platforming, there is no equal. I am literally stunned by the quality.
Valkyrie Profile - Wont get a better RPG on PSP
Exit - Fun and challenging puzzle based gaming, and usually 20 bux
Tekken Dark Resurrection - Stunning fighter that really uses the PSP hardware. AIs a little weird tho in the Dojo
Gradius Collection - Not owning this is dumb, its an amazing collection of awesome.