The available power saving and display modes in the canon firmware can also have a significant effect. In general, cameras with an optical viewfinder let you shoot with the display turned off, which also turns off the sensor hardware between shots.
Unfortunately, the G1X loses manual focus and focus lock when you close the door or turn off the display with the display button. It then starts auto-focusing again, which isn't good for time lapse. It doesn't do this when the display goes off from a time out (power save on). So I wonder if it would be possible to set the power save timeout to 0, so when the camera turns the display on every shot, it would immediately turn itself off?
Before you worry about battery life much, you should decide is if the battery you life you need is plausible. E.g. if you plan to take 24 hour time lapses, internal battery isn't going do it so you should focus on the ones that are easiest to use with an external power supply.
I agree that an external power supply is a must for time lapse. Here, the G1X is much better too. Power supplies with a dummy battery are around $15 on Amazon. The one for the G1X has a cable that comes out the side, so you can still use a tripod. The SX260 has a hole in the battery door that you plug the cable into, which blocks the tripod mount.
I also found an 8.4 volt lithium battery that works with the G1X by splicing the cable from a 2nd power supply to a cable that plugs into the 8.4V battery. I should get 7 hours this way without A/C power.
The SX260 has a 4.7 volt battery, so I did the same trick with a standard USB external battery for charging phones and iPods. I got around 7 hours on the SX260 that way. Without external batteries, both cameras get around 2 hours. Here's a video taken with both cameras. I had 3 internal batteries for the G1X, which I used for sunset the first night. I didn't even make it long enough to see many stars, which the sx260 went for 5 hours after it got totally dark.
http://www.youtube.com/watch?v=zkLBPlY6DRE#ws
