I'm sorry, but I personally think that a butterfly knife is a poor choice. If you need to use a knife, it usually means that you have let the problem get way too close. When someone is that close, I don't think it is a good idea to have a knife that you have to "flip" around to open it. If someone is relatively quick, they could get their hand on that butterfly before it is really open and that would present a problem. Another issue that I have with them is that they are difficult to open if you are lying on the ground and struggleing with someone. Even standing up, if you are struggleing with someone, they are much easier to drop (while trying to open) than a regular folder with a thumb clip.
Of course, this is only my opinion, but if I ever have to pull out my Gerber (slim, strong folder that clips to my pocket and is lightning quick to open with the thumb stud), it means that I don't have time or room to pull my handgun. That means I don't have time or room to have a knife fliping around my hand. My Gerber is faster to open...only a flick of my thumb.