feature wise, there is no difference. the combo has the same speaker outputs as the head, ie 1×16 ohms and 2×16ohms. meaning to say that the amp component of the combo is exactly the same as the head version, just a mirror image on the front panel. the speaker of the combo must be pluged into one of the speaker leads at the back of the amp.
if for instance, you happen to have 2 external cabs, the combo can also be connected to those directly, no problem. the good thing (and maybe bad, depending on what you want) about the combo is that it's one unit so transporting it will be easier. so if you like the sound of the combo's cab you should get it.
if you don't, then try the head with the extension cabs. don't bother with the 1×10" cab cause it sound almost the same as the combo, which is also spots the same 1×10" speaker inside. instead, try with the 1×12" open back and closed back cabs available at woodworks.
personally, i bought the head because i wanted to try it with verious other cabs (and i try to avoid 1×10"s cause i want more bass). currently i'm pairing mine with a blackheart 1×12" eminence cab, the bass is quite the killer! haha