I've come to the conclusion (based on responses here and personal experience) that when plugged in standby should be used and when on battery and you are away from your computer for an hour or more hibernation or shut-down should be used.
I would suggest the UXGA and if you are a FPS nut then drop down to 800x600 (still a native resolution 1600x1200 / 2) to get the frame rate.
UXGA is the best of all worlds IMO.