MBC for MBC, they both work the same. One fits better if you are only using the MBC.
You cannot compare a 2 stage MBC to a MBC+A. they are completely different. The 2 stage MBC will give you comtrol of 2 settings. This might be nice so that you can reduce your first gear MBC setting so not to completely lose traction and have a high setting for the higher gears. This design will not deal with the overshoot problems, but has advantages for duel settings over the MBC+A.
The MBC+A effectively deals with overshoots inherant in all ball and spring MBC's and the 2 stage MBC from Dawes will also have this problem. The other pro for the MBC+A is that it allows you to effectively move your boost tap downstream of the IC for longer sustained boost without droop.
So ultimately, a 2 stage MBC+A would be the best of both worlds. But...to do the 2 stage right, would involve more than just throwing on the dawes 2 stage. The ultimate solution would be to locate a magnetic pickup under the shift boot for buttonless switching of the stages. Without the +A, the two stage will still overshoots and you'll have to compensate for that.