EDIT: PSP VS the DS, i do have both a PSP and DS, and have to say that overall every game is lacking gameplay length on the PSP, you can beat almost any typical game/port in the time it takes the battery to run low (well literally some) the games look great but are way too short... it took me weeks and months to beat shonen jump super stars, where it took me a few hours to go through each mode on Naruto: Narutamate Ninja on the PSP.
all my games on the PSP like starwars BFII, Ghost in the Shell, Samurai Warriors: SOW, Tenchu,Death JR.,Mdievil,Rengoku,Darkstalkers, and even Metal Gear acid was too short for me... i like my games to take time to beat, and not worrying about playing through it in one day. The DS has only failed me on a few of those games... it doesnt bother me too much considering i bought all my PSP games for less than 10-20$ on ebay... that alone may tell you why?
a few example of some DS games i had to rid of being not currently worthy of my DS Collection because of gameplay depth or just plain too repetiive..
Sonic DS, (it was good but didnt have enough sonic-freindly characters)
Dragonball Z, SSW the modes were very few, and was written by a rap-artist
and Feel the magic, very short and way too repetative.
In conclusion id have to say if you want to play games, go get a DS, (or a 1.5 PSP motherboard ^.<) and go with a PSP if you want all that and a bag of Potato Chips...
btw, look out for Final Fantasy III on the DS, and Tales of Symphonia/Phantasia on the PSP