About the 12's...
I've used a 112 under a 500 combo and it kicks butt. It added a bark and throatiness that have been looking for for some time. That was when I decided to try two 112's with my 500 head. It kicks more butt. For my sound, the 12's just work better. I love a good 15 because of it's upper mid bark. The rumble 112's can have that if you want it. But they also have a lot of low end - so much in fact that, given the power rating, methinks it's possible to get into trouble if you dial in too much bass into the 12. Still, I've used the two 112 rig onstage now and it sounds so good, and is just the perfect size, that's it's my A rig. Honestly, I find it easier to move than the combo too - I'm always worried I'll knock the control panel setting it in the car. And I have indeed spilled water into it.
Either way, combo or 112 cabs, you can't go wrong. They sound different, the 112s maybe a little more open after the bass is backed down. I've had boom from the combo (210's), not from the 112's. The 210 cab sounds to me just like the combo. And for me, a 210 cab with the combo was too much of a good thing - I lost the midrange to the low end of the extra cab. But if you want gigantic low end, a 500 head with two 210 cabs (or 500 combo with one 210) is your huckleberry.
Good luck!