I'd say sleep. The only time I really shut it down is if it will be off for more than a day (eg over the weekends) or if it needs to be moved or unplugged. Sleep uses very little power, and its clear that it is designed to sleep as needed - you might notice that the blinking sleep light is actually dimmer at night-time than in day-time. The power consumed by sleeping for a whole day is probably less than that used in booting up the system.