For me--
1. Concealability. I don't care how reliable it is--if I cannot effectively conceal it, I won't carry it. This has excluded pretty much any double-stack from my inventory.
2. Reliability. It has to work, every time; otherwise, what's the point?
3. Ability to shoot reasonably well. This is where the rubber meets the road. My Kahr PM9 and my S&W 642 are similar in weight and size, and both are dead-nuts reliable. But I shoot the Kahr better. A bunch better. I can pick it up after six months' absence and shoot it better than I do the j-frame, after shooting the j-frame twice a month for six months. Five years ago, I thought that was a fluke, and sold all my j-frames. Eighteen months ago, I bought another j-frame. Guess what? It's still true. Which one will go out the door with me tomorrow? Take a guess.
4. Confidence in pistol/round. I have more confidence in a .45acp than I do in a 9mm/etc. I shoot a 1911 (even a 3" alloy framed 1911) better than anything else. So I carry a 1911 whenever I can do so. In practice, this turns out to be about 50% of the time. As much as I liked (note the use of the past tense) my two LCPs (and they were exceedingly easy to carry and perfectly reliable) I no longer own one. Right or wrong, I just don't trust the cartridge. The PM9 is only a bit larger, but I shoot it better and I trust the 9mm +p+ more than I will ever thrust the .380.
YMMV.
Whatever your decision is, make sure you base it upon objective criteria.
Best regards, Rich
Edited to add: You will note that capacity did not enter into the conversation (for me). While I understand that the day may come when I will face multiple determined attackers, I have done that calculation to my satisfaction. I hope I have done it correctly.