I'll go with an iPad. Sorry but handheld gaming is dead in my eyes. My PSP has been collecting dust a year after I bought it, same with my DS Lite. I'd rather have an iPad so that I can do other things with it.
I have both and other devices that connect to both.
I have PSVita and PS3 both with connectivity with each other making the experience that much sweeter. Loving the vibrancy of the Vitas screen and the back touch is amazing. Holding it just feels right and the joysticks have evolved handheld gaming. Bring a first person shooter ! Can't wait.
I also own an Ipad and Iphone 4 and the connectivity between them with Icloud is awesome. I use the Ipad for work by sending word documents to my email inbox when I write them. I can also get Remote Desktop with it aswell through an awesome application to control PC's around my academy
All in all I use them both about the same amount. Working with the Ipad and gaming on the Vita.
Typing this on my iPad right now. I fucking love this thing. I can't recommend it enough. It's got good games (infinity blade 2 is a 10/10 on ign) and typing is much quicker than I expected. Battery life is fantastic too surprisingly.