Self Defence?
Reliability. Of course. Looks don't matter when it comes to SD.
Overall though, I choose looks and feel on stuff. Unless you go for ultra-cheap, most guns will be reliable. Sig, S&W, Ruger, Taurus (Cept for parts),etc,etc. Most guns are reliable nowadays.
If I was down to two guns, both EQUALLY good, the looks would be the decider.