I play a tremoverb combo with 2x12 vintage 30's and a recto cab 4x12 vintage 30's i was lucky enough that i coudl match the speakers and im really really happy with the sound.
Pro, (not sure about the road king combo if it has 1 or 2 speakers) but the extra speakers the more sound.
Con, The power vavlue in a combo are not that far away from the speakers and espessially if yoru in a drop tuning the vibrations from the sound in the speakers can damage the power vavles over time. So a head is better in that way.
All in all you shoudl just try playing your combo through the cab and see how u like it, once again not sure about the Road King but on the tremo you can unplug the speakers so basiclly you can just unplug your combo speaker and jsut use the quad, and still have the other two for smaller gigs.
It comes down to prefrence i hope that helps.
Michael