There are different schools of thought on which is best. For example, some feel that the startup and shutdown processes create extra wear and tear, and therefore opt to leave their machines running 24/7.

According to Iolo Labs, which recently completed some studies on the subject, sleep mode wins the day (make that night). Their findings:

1. When you put your PC to sleep at night instead of leaving it running, you save $2 per month.

2. When you put your PC to sleep at night instead of turning it off, you save at least 25 minutes per month (in startup and shutdown time, I'm guessing), while spending only an extra five cents monthly.

I'm waiting to hear back from an Iolo rep on whether hibernate mode was considered in this study, as that would save you some startup/shutdown time and save you money as well.