Really, it is up to you. Lots of good points made already and it really comes down to what you are comfortable with and what you shoot the best.
If you prefer revolvers, don't get hung up over reload times. I've read some place that must SD shootings average 5 shots fired.
If you want a semi auto, do your research and buy one with a good repuation and then make sure its reliable.
Not matter which route you go, find some good SD ammo that works in your gun and practice, practice, practice.
And, as pointed out, they are mags or magazines, they aren't clips.
