Doesn't make much difference. I prefer sitting because I stay more stationary and can concentrate more on form. Standing to me is more awkward, and you might get tempted to cheat by using your legs to push up the weight.
why not do them both at the same time. well sorda. what im talking about is combing power clean and press. this kills both your traps and shoulders.
you have to use a little less weight than you can normally press. stand with the bar at waist level, dip down a little, (dont let the bar dip past your knees) power clean it up, and then press. drop the bar back down to your hips and repeat. i love this exercise.