A stiffer boot is will transfer energy more efficiently so it would be more responsive. It may be less comfortable but I have been in really soft boots that caused my feet to hurt way more than some stiffer pairs. However if they both fit exactly the same the softer ones would be more comfortable for walking around the parking lot and that.
I agree. And softer boots are better for beginners. They don't react quite as sharply as stiffer boots which will make your mistakes not have as many consequences. For example, if you get lazy for a second, your board won't turn and catch an edge quite as quickly with soft boots than it would with stiff boots. Same principle can be applied to board and bindings as well.