I often carry a 9mm with premium high performance ammo and don't consider myself undergunned . . . but I'm not convinced that it's just as effective as premium high performance .45 ACP ammo.
That's where I am. I wouldn't hesitate to carry premium 9mm. In fact after I move I'm trading my .40s in for 9mm. Yet I find it hard to believe that the same premium ammo in 9mm and .45 will be equal in both calibers. I think mass generally wins out, if only slightly.
I think the argument becomes do you feel better having more rounds available before a reload? A lot of that may have to do with what potential situations you think you may find yourself in. I'd definitely prefer a .45 or 10mm 1911 if I'm hiking in say wolf or lion country, to handle both man and beast, but might prefer 18 rounds of 9mm available in downtown Detroit. Then again, if I put myself in a situation where I need the 18 rounds (or a situation where I'm facing a cougar), I probably should have taken a rifle.
Though the "9mm may be easier to shoot [accurately]" is an extremely valid point. I, without trying to brag, am pretty darn good with the 1911, which is why I don't hesitate carrying it. I will note though, that in every defensive pistol class I have taken, I was the only 1911 guy. The instructor I used carried a Sig in .45, and I remember one class where a guy shot a sweet S&W Model 19, and I wouldn't have wanted to get into a gunfight with him - I wouldn't survive. Everyone else shot high capacity 9mms (there may have been some .40s).
