Usually I am just around the house so it does not really matter too much. I will just plug it into the wall. But it takes me about an hour to get to Chicago on the train, do my business, then an hour coming  back. Unless you are for some reason not near a wall plug it should be no problem.

That was one test. At the minimum settings test, the PS Vita was fairly close, the 3DS lasted just over 20% longer. But what about maximum settings? Rolstoppable pointed out that I overlooked the speaker and wi-fi power consumption, but still, the biggest drain on power are screens. What's going to consume more power? Two LCD screens, one of which is 3D, or an OLED screen? I can't find any data on the power consumption of the Vita and 3DS screens, but OLED screens usually consume a quarter of the power of LCD screens, let alone two of them, one of which is 3D. 

If you crank up the backlight, the 3DS's battery life is going to take a significant hit, while the Vita's will only take a small hit. If you compare minimum and maximum backlight on both systems, as well as 3D on and off, I think the 3DS will last longer on minimum settings but the PS Vita will last longer on maximum settings. I have no idea how they will perform with wi-fi turnd on and speakers turned up, so like I said before, I want to see more comparisons first.

All I can tell you is that whenever I play the 3DS the wifi, 3D, and volume are always on. I tried to do a battery test with everything on and brightness at 4 but I got bored pretty quickly. What I found out was that the 3DS lasts about 1 hour 25 mins or so each bar, which comes out to above 5 hours. (I think.)

5 hours? Really? That sounds fairly reasonable actually. What game were you playing?