Here is my take on the subject. It's dumping in Utah and I rode waist deep pow yester day with my slalom set up, and another day of waist deep today with my soft powder set up. With that beining said I prefer my soft set up but not by a whole lot.
Some reasons for this are, flex. Yeah you can flip a switch on a hard boot and loosen the cuff, but you only get a piviot point at the ankle, a soft boot flexes through out the whole boot including the sole. Another is on a soft set up you are much closer to the board with you feet this help get more feel and less leverage; in pow you are using more subtle movement with your ankles and a soft boot is better for this than a soft hard boot. Because you can get so much leverage on a hard boot set up I useally over edge in pow, not so in my soft boots.
This is why I prefer to ride pow in soft boots, even though my hard boots fit better and are way more comfortable after a day of riding in them. But if I didn't have the luxary of having many different set-ups I would pick hard boots for every thing.