I think from a legal aspect, when you are sitting there facing a hostile district attorney, you probably would MUCH rather have to explain having only to fire one or two .45 rounds to stop an assailant rather than emptying a clip of 17 rounds of 9mm into the creep.
Now if you had 5 bad guys down for the count because you had the hi-cap gun at hand, then no problem with the DA, I would guess. But one guy with 17 holes in him? No matter how right you were in your actions, you will be sweating the potential legal aftermath.
IMHO, of course.