depends on many factors.

assuming everthing else is equal- if he has better coverage on ATT than you do on VZ then his phone has to use less power to transmit to the cell tower and his battery will last longer.

I know one flavor is better at battery life than the other- so could be GSM is better than cdma.

could also be that there is better firmewar on the epix. We're still waiting on the first firmware update from VZ and seems we should already be on the second one had they not kept rejecting them