I voted cartridge, but let me tell you why. My first pistol, and my primary carry gun to this day, is a G23. I wanted a .40 becaus I thought it was significantly better than a 9mm but still allowed high capacity and a compant frame. I picked the G23 because it was lighter, had higher capacity and more readily available components than its competitors. On the other hand, I picked the Kahr platform for my small gun and decided to get a P9 covert because of it's lower recoil and slightly higher capacity. I was coming off a 340PD and I wanted to make sure that I would want to practice with this gun. Both worked.
I don't think it is possible to divorce a gun from the cartridge it fires. Changing calibers completely changes the "decision matrix" that I use, which weighs weight, size, caliber, capacity and mode of function.
Which is a better fighting gun, a traditional 1911 or a G20? The G20 has a higher capacity and is more powerful, but the single-action trigger may outweight these advantages in some peoples' books. would these same people be willing to use a .22 1911 over a G20? Would a traditional 1911 fan go for a G21 over a 9mm 1911, choosing caliber over platform? Caliber changes so much in terms of the realistic application of a pistol that it can't be removed from the "type" of weapon in the decision making process. The caliber is integral to the design of the weapon, so I believe each weapon must be evaluated on its own, not in terms of "45s" or "polymer framed" or "double action." To make decisions purely on an arbitrary bias toward a caliber or classification of a platform is silly and unnecessary.