I say get both! Save up extra money and get both! Both have some brilliant games out and coming out.

PSP has FF7: Crisis Core, God of War, Final Fantasy Tactics, Valkyrie Profile and will get FFXIII Agito as well as a new Parasite Eve game. Not to mention homebrew stuff is brilliant, especially emulating PSX games.

DS has some awesome stuff too, like the Final Fantasy remakes, heaps of cool Mario games if you like them, Pokemanz, etc. And it can also play GBA games, which you can also homebrew and emulate.

So yeah, get both.