I have the GK MB210 (the kid brother of the MB212 yer lookin' at). It's a great amp, but with a separate head… I could match purt'near any speaker combination I want with it. Also… on the combo… the controls are at the rear of the cab, and facing skyward (not the most convenient), and if you stack it on an extension cab, it could be a struggle to see them at all. Also… stacking/packing/portability is better. All that being said… my MB 210 is small enough and light enough to be carried with one finger. As it sits… I get 350 watts out of the 2 onboard 10s. When I ad a cab & run at 4 ohms, I get 500 watts. Also worth noting, is that GK makes the MBP series cabs that go nicely with these combos. The MBPs are an external cab with its own on-board power, that run off the preamp on the combo. If you were a little power-mad… you could add a 212 MBP cab to the MB 212 combo, and have a 1000 watt 4x12 rig!
GK did a great job of building versatility into their stuff, so at the end of the day, it pretty much comes down to your budget, and what's most convenient for you