Personal opinion will be effected by the type gun you shoot, the way the gun feels in your hands, your ability to control the recoil and a few other things.

I have a Glock and a S&W in 9mm. I would rather shoot the S&W.

I have a Glock and a S&W in .40. I would rather shoot the Glock.

I have a Colt and S&W in .45. I would rather shoot the Colt.

For self defense, I feel more confident with the .40 followed closely by the .45 but would not hesistate to use the 9mm if need be.

True enough, the 9mm is cheaper to purchase, cheaper to shoot, somewhat easier to carry and has more capacity. I find it fun to shoot but then I like to shoot my S&W .22 as well.

Just as correct is the fact that those defending the 9mm are those that pretty much only owns a single handgun and that being a 9mm. Never offer to swap them a larger gun even because you will be out a larger caliber and they will then be on the flip side of the 9mm.