Well, of course, you can resume from 'Sleep' very quickly and 'Off' takes considerably longer.
'Sleep' uses some battery power and 'Off' virtually none.
When you power down and power up, the iPad does quite a lot of 'housekeeping' and 'cleaning up', so Apple recommend you do that every so often. It solves a lot of 'issues' too - apps that quit unexpectedly, for example, or 'misbehave'!
Tim