Re: AFCI-12/3
For two BR's in close proximity, the 12-3 is less wire to install and staple up and one less neutral cluttering up the feeding panel.
But, using a 12-3 rather than a 12-2-2 would mean using a pricy 2-pole AFCI compared to two singles.
The 2-pole units (GE and Cutler) are useful for retrofit work where you've gotta work with what's already in the walls, but less interesting for new construction because of cost. The GE's aren't really all that bad if you wanted to use 12/14-3, but the Cutlers price difference is pretty bracing.