Excellent replies so far.
I like using a combo amp when I'm on a big stage with FOH support and there are other acts on the bill, causing me to have to hustle quickly on and off stage.
I'm always a bit obsessive about bringing two of everything, in case something breaks. (This has almost never happened, but I do it anyway.
) In the scenario described above, the FOH support is there if the combo amp fails. For every other situation, I like to bring a cabinet and two heads. Having two heads also gives you flexibility with your sound, depending on the acoustics of the venue. In time, you can build up a collection of heads and cabs to mix and match depending on the acoustics, the music, what bass(es) you're using, etc.
So, I'd recommend the head and cab option, and don't skimp on the quality of the cab. As others have said, you might be able to find an affordable used one made by one of the many major non-boutique companies (Ampeg, Gallien-Krueger, Fender (the Rumble line is good), etc.)
Also, make sure you have a good speaker cable! I like 14 gauge weight. Buy a couple, and take good care of them.