This all depends on your model of the gun fight.
Is it the one mugger scenario?
Is it the rare multiple target scenario or even a single attacker where the shots are hard and you need more than one? Of course, if you need to fire more than one, you ain't doing your job, boy - blah, blah, blah commando, blah, blah.
That being said if you think that it could happen that you need a lot of ammo, then you need a lot of ammo - thus given that quality 9mm is as efficacious as 45 for most purposes - go for that.
At the NTI, when we were given J frames and four bad guys - ammo issues were a touch exciting.