I think it's more of a personal prefence than anything else, and there's some other stuff that has to be taken into consideration, like details. For example, you can take a decent 15w practice amp from fender or gibson or ibenhad and I can guarantee a decent stack is going to rip its balls off and skin them. On the other hand, you take a bad cat 15w or a carr 12w and PA it you're probably going to have way better tone than if you just blast the place with marshalls. A 15w hot cat model from bad cat might even have way more/better gain than some marshall stacks (can't say I've ever owned either).

My view is similar to guitarman's except I haven't had the pleasure of playing guitar (or being alive) that long.