I agree that standing is my preference b/c I find my form is better when shooting from standing rather than sitting. But my buddy talked me into practicing my shooting form from the sitting position. Not so as to always shoot from the sitting.........but in case a shot is offered while sitting and to stand would spook.

I split my time between standing 75% of the time and sitting 25% of the time (approximately). During the magic twilight hours of dawn and dusk (you know, those 25 - 35 minutes of partial light when everything seems to stand still and move at the same time) I will always stand.
There are 2 advantages to sitting though. #1 is it gives you a chance to relax your muscles so as to be more relaxed when the time for the adrenaline rush comes. #2 You can sometimes gain a new perspective, ya' know.. a different angle of view. Every time you move you risk being spotted, but I have been able to view a deer's head when sitting when otherwise all I could see was legs when standing. Soeetimes I prefer to have the deer's head obscured so they can't see me either, but at other times I need to evaluate the deer for shot decision.
All in all, though, I prefer to be standing most of the time, especially when the shot presents itself.