For what it's worth, what you don't need for defense is a nice knife. And you don't want it hair whittling razor sharp either. Think 'one shot deal'here. You're going to loose it to the police, or ditch it in the next dumpster you pass, if you end up using it on someone. Depending on the circumstances.
Yes, I'm sure the Spyderco's and Benchmade's and the other fancy knives are good knives, even may be very good knives, but they are over kill for what you need on the street. Look at what most street criminals use, there's a method to their madness. Quality of blade does not even come into this. We're not field dressing a buck here, just slashing or stabbing a criminal to get them off you, or buy you some space to get the heck out of Dodge. The knife that will skin out X number of deer before needing to be sharpened is not needed here.
The street punk will use a cheap serrated edge paring knife or other kitchen knife he stole from a discount store, and he may even have a cardboard and duct tape sheath for it. For slashing and stabbing a human body, it's fine. The human body has about as much resistance to being cut or stabbed as a bowl of noodles covered by a layer of cloth. That's not much. Anything made of metal and pointy will do the job. For sharp, you just want it 'prison shank sharp' like in stropped on a sidewalk or cement floor. If you have it really sharp, the cut will not even be felt right away. Think about the last time you cut yourself with a really really sharp knife, and it didn't even start bleeding for a bit. That's not what you want. you want the criminal to feel it immediatly, to feel pain. A razor sharp edge will neatly slice and not tear the tissue being cut. A course ripping edge is what you want. This can be put on any gas station 4.99 Spyderco knock off. If the knock off has some nice big serrations, even better. You want ripping, not neat cuts.
I remember one call we rolled on when I was with the Trinidad PD. A lady who was routinely beat up by her drucken husband had enough finally. In the middle of a beating, she grabbed a 1.99 cheap serrated bread knife off the kitchen table and rammed it into his big beer belly. Dropped him right to the floor. We got there and before the EMT's, and he was laying there moaning. The blade was so thin it wobbled every time he breathed in or out. Was weird to watch. Breath in, wobble wobble one way, breath out, wobble wobble the other way. But it did a number. Several hours of emergency surgery and he lived, but it was a close call. I gained a lot of respect for cheap knives as a police officer. Of the couple of homicides I handled as a police officer where a knife was the weapon, I never saw one that cost over 10 dollars. One stands out in my mind, because a cheap Rapala fillet knife was used. It slid right in between the ribs and into the heart. Death was instant. The Las Animas county coroner said he was dead by the time he hit the ground.
If you want a nice knife like you'd rather drive a Lexus rather than a Toyota, that's okay. But don't think a good knife is needed to inflict massive harm in a defense situation. Any piece of sharp steel will work just fine. It won't matter if it's a 100 dollar Benchmade or a cheap 5 dollar Chinese knock off of some tactical thing.
It's more important to know what your doing with, it than what the tool is.
The modern martial arts dojos don't really give good training on knife tactics. I wasted a couple years in Tang soo doo before I quit. The commercial studios just teach the standard defense from over head ice pick grip stuff, not the real street in your face stuff that you'll run into. The Philipino martial arts are better for bladed training.
The most important thing on the street is to just keep your eyes open. The street punks always work in at least two's. One will have a knife, and one will hang back acting as lookout. If you're not sleep walking, You'll see it coming, and that's the time to take out your own knife, even if to 'clean you fingernails' with it. Like a firearm, the sight of a weapon in a would-be victims hand is very very often enough to make the punk look elseware. Predators are looking for a victim, not a fight. They don't want to get hurt, and they have a very good idea of what a knife can do. I know there's all kinds of internet guru's preaching don't show your weapon. Bull hocky.
I can only go by my own experiance as a white haired old fart high on the predators hit list profile. Twice in the last few years I've had the would be predator back off from me because I had a knife in my hand. One time was I was stumbling along in condition white, and didn't notice the ambush untill almost too late. All I had was a ridiculous little Buck Hartsook in my hand, and some very harsh foul language. It was enough. The other time, I had a guy that was two of me, maybe three, but he didn't want to get in arms reach of me and a very humble little Opinel number 8. Both times I went on with my life because they backed off from a white haired skinny old guy with a knife.
The kind of knife you carry won't matter a bit. It's attitude and what you can do with it. There's guys in the cell block that if they have a home made shank, they'll put away a yuppy commando with a high end tactical.
Like has been said, software over hardware.