depends what you want. If you want a robust gaming experience designed by someone that hates you, get the PSP. If you want a relatively weak gaming experience backed by a ton of nostalgia and franchises literally worth more than the PSP, get the DS.
Well it has to be a 3DS. I've owned a PSVita and a 3DS and I ended up selling my Vita because there were never any games created solly for except actually PS3 games made portable which is sometimes a good thing but some games were never meant to be portable.
Unlike the 3DS which actally takes time to make personalized games for it as apposed to making Consol games into protable one's. Thats how I see it.