If its actually for self defense, i.e. you may have to stick someone who means to kill you, I wouldn't carry anything smaller than a 5" blade, and even that's pushing it a great deal. There is a reason every combat knife ever made to be carried by a soldier is considerably longer than a pocket knife. Short blades don't hit vitals and don't kill. Long ones do. The Bowie knife wasn't a fad because it was big and kewl looking, it was popular because it worked very well as splitting BGs open, slicing their arteries, and puncturing their delicate parts. That would be the .44 Magnum, but for pocket carry think .25 as opposed to 9mm or .45. Anything small is simply too underpowered to depend upon, but just like a .25, its better than nothing...
Many of these "self defense blades" are really no better than a prison yard shank, because they are quite simply too short, and you would have to use them exactly like you see the prisoners doing it. If you're going to carry one of those for self defense you have to be prepared to use them thusly, and my guess is that very few law abiding people have that kind of ferocity and violence pent up inside ready to explode on a would be criminal...
There are allot of long folders out there for under $100 in cheap 440c, AUS8, 1055, and other high carbon steels. Really any of them will work, and it just depends on what's most comfortable. I carry an XL Vaquero, with a 6" blade, but they don't make them anymore. They really don't have to be all that tough if you intend to cut manflesh with it, and even cheap Italian stilettos work well (they've been murdering each other with them for hundreds of years).
I absolutely love knives, all of them. It's ridiculous how many I have, and there is no question that the models and brands mentioned are great blades. However, carrying one to cut stuff and clean your fingernails isn't the same thing at all as being committed to actually using one for self defense. A knife is considered a thugs weapon almost everywhere except the military, and using one to cut someone has even more dire legal ramifications than shooting someone. It also can be a hell of a lot more dangerous. As a self defense weapon they require a whole different mind set, and though the laws are the same you will not be treated the same if you cut someone. It will be allot harder to convince the police your the good guy in a stabbing rather than a shooting, and that's just the way it is, because of the type of people who are most often involved in stabbings.
My $.02.
