Is one better than the other or is it more of a weight issue?
I.e. Peavey 6505 combo vs 6505 Head with 2x12
If it's a tube amp, a combo can sometimes subject the tubes to a lot more vibration, which can shorten their life. And with tube amps, it's definitely a weight issue. But with separate head and cab, you get more freedom to have different cabs for different situations, or fine-tune the speaker and cabinet choice to your playing style.