Isn't this really two different questions? Low power vs. high power and head/cab vs. combo?
I think you are spot on there Philly. The OP definately seems to have got these two issues mixed up in his mind.
For home use (ie. low volume) I don't think the head/cab rig has the main advantages that it would have for gigging - which are :-
1/ distances the tubes from the vibrations of the speaker cab at high volume, reducing tube microphonics.
2/ Allows a larger cab to be used, or multiple cabs (although this is, of course, also possible with most decent combo's too).
But a head/cab is also just as appropriate for home use too. Let us not forget that cab's come is all sizes from 1x12 to 4x12.
For sheer minimum size and space, a small combo cannot be beaten. However, some combo's are very powerful , very big, and very heavy - these are much more oriented towards players who play live (although it takes a bigger man than me to lift a 2x12 AC30 or Fender Twin for any length of time).
As for power, and suitability for home use, it all depends on the design of the amp. High wattage amps which are designed around pre-amp gain will often sound just as good at home volumes. But amps whose tone is dependant on some power tube break up or compression (eg Ac30 or Vintage Modern) need to be much, much louder before they start to sound good. An Attenuator is a must for home use ( and also usually for gig use) with these amps. Remember, if you use much gain at all, any amp over about 20W will be monstrously loud over about 2 on the volume.
Attenuators and power-scaling are your friends !!