Restart's quicker by the amount of time it takes you to push the button for a cold start, and the spin-up time of the disks, because they lost power. (they stay on in a restart).
You can use restart (or iReboot) and leave the room. Shutdown makes you hang around to provide human input.
I have heard that if you hit start before your disks have stopped spinning, that it can cause hardware problems (possible head crash), but I suspect that's an urban myth !