I use heads and cabs for gigs exclusively, both 2x12s and 4x12s. I have a combo for rehearsals which is stored there.
Tonally, I've found heads and cabs to be superior because the cabinets are usually nice and solid. If you try and instill that into a combo, they end up weighing an absolute ton. My 2x12s are semi open backed and are less directional than my closed back 4x12, but still have a nice big wallop in the low end.
I guess what I'm trying to say is that neither option necessarily sounds any better than the other. It's all about the convenience of the package that suits you best.
Dave has covered it there ; the bits I would have written if Dave had not prompted me are :
Both have merit, but for the 'main feature' I liked seperate head and cab .
The cab
is usually built like a tank, yet very portable ( if you only have a lift in a small car for a BIG gig ).
A solid cab makes a great' base' for the speakers.
For tonal experimentation the mix & match is quicker. You quickly plug into your mate's cabs and visa versa . If you are experimenting with different speakers within the cab , you can then try it with every amp in sight.
You can have the amp facing you / side stage and up at face height for quick adjustments, when prompted by the sound man. Ditto for the cab - facing away / across the pub/ stage / getting sweet revenge on the drummer.
Sell one, keep the other if needed ; or lend / borrow a 1 x 12 / 2 x 12 / 4 x 12 to accommodate venue / style / room acoustics.
Flexibility.
:)