To be honest, I'm not a big fan of carrying a knife as a defensive weapon. Unless you have extensive training in combat with edged weapons, and you feel that you're highly proficient at it, and probably even then, you're going to get pretty cut up in a knife fight. If you aren't well-trained, you're probably going be at best get disarmed, and at worst, get yourself killed. I spent years in martial arts both in the US and in Asia, and trained with ex-military SF guys for knife combat and I still dislike it greatly. My humble suggestion to you is to always carry a knife, but carry it as a tool. Knives are fantastic and indispensible tools for every day use. I carry a Spyderco folder, myself. (Gibbs rule #9) but if you want something other than a firearm for defense, depending on your state laws, consider an alternative like OC pepper spray, a stun gun, or an ASP tactical collapsing baton (which also requires some training to use properly, but gives you a lot more reach and distance from your adversary as well as a lot more momentum at the end of the baton on impact.)