In a head-to-head battery test, the Nintendo 3DS outlasted the PlayStation Vita. YouTube user Kurekureyon filmed himself comparing the battery life of the two competing handhelds. In the video below you can see the two separately timed play-sessions.
To ensure that both systems would get the most battery life, the systems had their screens dimmed, volume minimized, wi-fi disabled, and 3D turned off (for the 3DS obviously). He played F1 2011 on both systems, timed the tests, and recorded the results. If you're too lazy to watch the entire 9 minute video, just skip below for the results.
As you can see the PlayStation Vita dies just before the 4 hour and 4 minute mark. The 3DS, or I guess DS since the 3D was turned off, died three minutes short of 5 hours.
While there are some discrepencies with the testing, such as turning off the 3D, I think you get a good idea for the overall lengths of each handhelds. Subtract about 30 minutes for the 3DS if you play with the 3D turned on and you end up with the Nintendo 3DS still finishing ahead - but barely.
Either way, both companies were accurate when they promised 3 to 5 hours of battery life. And if that's not enough, there are plenty of external batteries to extend your battery life - like the Nyko Power Grip.
So the Vita beats the 3DS on highest setting and the 3DS wins on the lowest! It's a draw I suppose