It is a no brainer everyone is going to tell you PSP, it isnt their money. Go for the PSP if you have the money, but i havent seen any WOW titles for it, unlike for the DS.
PSP, if you can afford it. Why? Browser Guilty Gear Street Fighter Burnout Wipeout Mega Man Powered Up.
PSP mate, the games are much better (graphics ) but if you want some kiddy good gameplay you can buy dslite
I'm going to be a trend breaker and suggest the DS. The main reason being the fact that their arn't any titles out at the moment or upcoming releases for the PSP that seem interesting to me. Sure it has alot of options, but doesn't seem to be going anywhere (game-wise) IMO. Although i'm waiting to see how it will interact with the upcoming PS3. If you have the money to spend, get both - that way you can't go wrong.
I bought a PSP my brother bought a DS. He always want to play on my PSP that explains enough I think!!
Lucky you! You've got the best of both worlds I'm hoping E3 this year releases alot of information regarding these two handhelds (preferably information about the PS3 and how the PSP will be able to interact with it).