Unless you have the time to spend at a range, just get the strongest mace you can find, even better if it contains UV dye. Put it on your key chain A- so you don't forget it, and B- so you remember to take it out when you walk to your car.
I recommend mace over a gun in large part because, without practice, you are less likely to hesitate to use it if you have to.*
If you do use it, run while he's stunned, then call the cops. Tell them you sprayed him with mace and that it contained UV dye so they can identify him and nix any bullshit about "She's lying!"
*Although a shotgun under the bed for home defense never hurts.