Since 1985 my only carry piece has been a 1911 Gov't model .45 in a Bianchi 3s IWB. I just dressed around it. Lately I've been carrying a 39-2 clone in an old Safariland Model 28 high ride holster. I just work my wardrobe around it.
I don't switch from auto to revolver and back because all my formal training has all been with a 'Bottom Feeder'. And I know for a fact that in a stressfull situation 'SHTF' you will revert to your training as an automatic response to the situation. I would be lost with a revolver. I have several and love shooting them all but I still handle them as if I have two left hands wearing gloves. I guess to answer the question as to 'What decides which gun?' It's the one I know how to use the best and have the most confidence in.