Why would you buy a DS when the psp has limitless possiblities one being emulaters like snes, nintendo 64,sega,FBA ... the list goes on. also you can have custom made software on it. And why would you buy games when you can easy download the iso and put it strait on your psp card. It is easier to and cheaper to get your psp done than DS.
the only thing you can do with a DS is put DS and GBA roms which i find pointless seeing you can already put gba roms on the psp. and putting music? why would you want to do that iv heard music on the ds it sucks to be honest.
If you do end up getting a DS get a Supercard not an R4 i find them better. But this is just my oppinion offcoures.