Back in the days when I was using a Head + 4x12 setup I always had an 'A' type top cabinet, thinking that it was a better look for the half stack. Many, many players will say that the 'B' type bottom cab is the better sound if you are only using one. Never had the chance to compare but I would guess the 'B' cab may have a little more bass with the slightly larger internal dimensions.
Marshall DSL401 Combo
Ampmaker 18w Combo
Orange Tiny Terror 'Hard Wired'
G12C 2x12 Cabinet
Vox AC30, Sound City 50, JTM45, JMP50, 9004, DSL50, 9001, EL84 20/20, Class 5, 2266