In order:
- Can I shoot it well?
- Can I carry it using the methods that are acceptable to me and to the law?
- Is it reliable?
- Does it carry enough ammunition to give me a fighting chance against two attackers with typical miss rates and factoring in the typical effectiveness of handguns in this general performance class?
Why in that order?
I won't consider a gun for carry unless I can shoot it well. It doesn't make any difference how well it conceals or how reliable it is, or how much ammunition it carries if I can't hit the target accurately and rapidly.
It doesn't matter how reliable it is or how much ammunition it holds if I can't carry it. I will never consider a Glock 20 as a carry pistol because I can't conceal it. The facts that it's reliable and very effective don't enter into the equation because, for me, it's not a suitable gun for carry/concealment due to its size.
If reliability were hard to achieve, this might move higher on the list. As it is, there are a number of reliable pistols out there. So I can move it down the list with confidence knowing that once I've achieved shootability and concealability I will still be able to find a reliable pistol that meets my needs in the other categories above this one.
Capacity isn't the end-all, be-all, but I'd like to have enough that I'm not relying on the gun primarily as a talisman. I have it on reliable authority that it is hard to hit a target that's moving and shooting back, and if missing is a real possibility then I think it's important to have enough in the gun to be able to miss a few times and still have a chance of scoring two solid hits on two opponents with what's left in the gun.