a 212 is louder for same input, even if the total cab impedance is the same (which would draw the same power from the amp as the 112 of the same impedance). More driver surface area makes for more volume.
However, many solid state amps (not all) deliver significantly more power to 4 Ohms than to 8 Ohms. For examples many class D 350 watt heads actually deliver between 200 and 250 watts into 8 ohms, and only deliver 350 watts into 4 ohms. If you have a single 212 cab running at 4 ohms, you're getting the most out of it in a single cab, and also volume wise the same surface area as two 112 cabs, so it's great.
But, a 212 is much heavier than a 112.
So I, and many people, buy 2 112 cabs. My 112 neo cabs weigh about 25 or 28 pounds each, which is shockingly light to carry for a speaker cab.
Mine are 8 ohms, so I can connect two of them to one amp (which be default is in parallel connection on most amps), and voila they now run at 4 ohms system impedance and the amp chugs out 350 watts (if I was using the head I described, I'm not, but I used to).
So 2 112 cabs is lighter and smaller to carry one at a time than 1 212. MUCH lighter and smaller to carry.
I like that. I'm not good carrying big cabs, and as soon as you hit 30 pounds I no longer can carry anything very far, so that matters to me also.
Sound quality, if the cabs are both designed properly and with the same goals, should be very very similar.