I'm about half and half. The sleep is meh for me, my battery takes a good hit if it is unplugged overnight. I have a momentus xt so the boot time is about 20 sec so there is no way that is going to us more power than sleeping it overnight.
overnight - shutdown>sleep
~2 hours - sleep>shutdown
I'm in bootcamp a lot anyway since apple can't figure out how graphics work. On a side note its surprising how much quicker/snappier win 7 is compared to os x on the same machine...