On this table are 21 handhelds, including the Vita, PSP and just about every flavour of Game Boy, DS and 3DS there is/was.
The test is simple. Start them running, and see which one runs the longest.
It's not entirely scientific, as it's slightly unfair to compare the battery drain of a PSP game to a Game Boy one, but then you could counter that it's all relative, and that I'm not a scientist, and that I should just shut up.
A fair point.
For the impatient, the overall winner was the Game Boy colour, which lasts so long it nearly made an appearance on Life After People.