In terms of battery life alone there is no contest. I can pick up the DS on and off to play mini-games and the battery will last me a week or so. But that is also another point, that's all I use the DS for! Wipeout Pure and Lumines are both very addicting games. Lumines I play when I'm home and Wipeout I play alot via WiFi with my neighbour and at school.

I wouldn't worry too much about battery life though, both the DS and PSP use a replacable battery (Although the PSP one is much easier to get to). They cost about 30 dollars? (Haven't looked at the exact prices but I recall seeing a 30 somewhere, the DS batteries are cheaper) And you can swap em once one runs out.

The only time I even looked at battery life for ANYTHING, was for my Powerbook (Their batteries are about 200 dollars CAD), and iPod, because it's a bitch to replace those.