here's the thing,
ps3 controller is rechargeable, xbox controller is not, so you have to buy batteries, so depending on what batteries you buy, that determines the battery length.
if you buy rechargeable batteries for the xbox, again, it depends on the brand.
eventually all batteries will stop working to full capacity. in which case, xbox ones, you can just replace, however, ps3 ones you cant.
for example, ive had my ps3 controller for years now, and it lasts me about 2 days of gaming. while my xbox, ive had for not long, and it lasts me for days.
I'd vote PS3 because you don't have to buy anything extra to make it rechargeable it's just that way out of the box. As for which hold there charge longer I really don't know there probably both about the same.
I own a Wii. What is this thread about? JK, well I use to but....on to the OP
Well, I own both PS3 and Xbox and I'll have to say the PS3 controller has a better battery life than the Xbox.
PS3, if battery is low, you can plug it in and play as it charges. Even if you get the charging dock for 360, you still have to wait for the battery to charge before you play again.