Allow me to answer by way of a mnemonic: hibernate is great. You see, sleep mode (a.k.a. standby) puts your system into an off-like state, allowing you to pick up where you left off after just a few seconds (unlike rebooting, which can take minutes). But a PC in standby mode continues to consume battery power, so it's not uncommon to return to a "sleeping" PC to find that it's just plain dead.

Hibernate, on the other hand, writes your machine's current state to a temporary hard-drive file, then shuts down completely (much like "off"). When you start it up again, it loads that file and returns you to where you left off--no booting required.

Both ends of the hibernate process take a little longer than standby (usually 10-20 seconds, in my experience), but you avoid any of the issues that can arise when Windows suddenly loses power. What's more, standby is a notoriously flaky mode. I've encountered plenty of systems that refuse to wake up properly, so you end up losing whatever work you were trying to preserve.

Consequently, unless you're running your laptop on AC power, I recommend using hibernate most of the time.

And here's a handy related tip: You can so that pressing it automatically activates hibernation.