The PSP is great. Mine has GBA/NES/Genesis/SNES emulators on it, which allows me to relive Christmas 1991 whenever I feel like it. The simple UI makes it easy to store songs and files where you want, too.
The Vita, however? I've got one, but it's not great at all. Sure, New Game Challenge Stage, Persona 4 Golden, and other Vita exclusives exist, but the inability to simply "plug a wire into Vita and PC to move files around" is a great detractor for me. In Sony's effort to create a device with anti-piracy, they ruined a system I really wanted to enjoy.