For me the advantage of keeping it in sleep is having all the apps open and exactly where I left them. "Session save" type features never keep things quite right - some apps just don't reopen, they're often not on the right workspace etc, not to mention documents and so on have to be saved if you power off.

You can of course use hibernation to get the best of both worlds, at the cost of long start-up times, and so I do often do that, when I'm not expecting to turn back for a while.