I don't think there is an answer to what kind of firearm is best. It's whatever you can hit with and have practiced like crazy with, caliber won't matter if you miss, neither will speed, cause you won't be able to miss fast enough. It's whatever you can hit with.
Backups...I personally do not carry a back up, some do.
How I carry? Either IWB or OWB, same holster does it both, and after many years of trying lots of holsters I have a box full of them, I finally found "the one" I like best. Everyone is different with different shapes and weight.
What I carry? I carry a .45 cal 1911A1 most all the time, in some rare cases when it's really hot a small double action revolver, but most the time a 1911. One spare mag. Why? Cause I can hit with it and that is what I've practiced with the most as far as defensive practice, it's not because it's light and easy to conceal. (see above, a box full of holsters before I found one for me) There are better choices for concealed carry, but my confidence lies with what I shoot the best, and what I probably will shoot the best with if I can't think straight and am in a pile of trouble. If I decide to carry something else someday as a primary weapon I would need to spend lots of time with it first so there would be the same confidence.
You'll get lots of replies on this one, but the important thing is, if you carry a gun, be good with it, practice all the time, and if you can't get to the range a lot, practice presenting your weapon from it's concealed holster and dry fire it. When the finish wears off of it from repeated drawing and live/dry firing , you'll be able to do it without thinking, which is what you'll need if something bad happens in a hurry.