There are a lot of factors in power consumption. Monitoring the power that individual apps and services use is a start, but you should also look at environment and patterns of use. For example if you and your GF both have three email accounts but hers is primarily gmail that pushes new messages to your phone, but yours is a POP3 account that you poll every 5 or 10 minutes, even though you have the same apps running you will use more power. If you normally carry your phone in your pocket and your GF carries hers in her purse, the temperature difference alone can effect the battery discharge. Of course, these will be minor, but put a few together and one phone will look like a miser while the other is a hog.
