Personally, PSP Go hands down has the best design for handheld.

It really showed the PSP as a true gaming device (when slid open) and multimedia device (when closed).  It has a nice small size that actually feels portable in your pocket and I love that it hides the main buttons/d-pad/thumb stick so it protects it from getting particles inside when it's in your pocket.

I would love it if the Vita had the same re-design treatment as the PSP Go, the media feature would be alot better with a touch screen.

Any unbiased opinion would lean towards psvita, its design is pure genius.
Personally, I didn't quite like the Dual Screen technology, even though i spent big portion of my life playing the old DS.

Vita's design is nice, though my main complaint is that the speakers can easily get blocked by your thumbs..

I really like the DS/3DS design because it works well as a clampshell

PSP Go is the best looking portable I've ever seen. PS Vita is a very strong second. But I also have much MUCH love for the GBA Micro!