Carried a knife from the age of 16. In school.
Had more problems with carry the last few years around airports and courthouses, although the budget battle the Sheriff and County Commission had about who was paying for staffing got the xray machines pulled finally. Good riddance.
Most of the time it's been a locking folder in the 3 1/2" range, other than that, there is without a doubt no effective difference in terms of self defense. It would take an expertly trained and skilled individual to use the slight difference. What counts going in is knowing You Will Get Cut Up No Matter What. If it's that desperate a situation, you should have pulled your gun, and used it.
Knives in self defense lack stopping power in a much higher degree than a bullet. You can't stab or slash someone and expect them to fall down incapacitated, it will not happen. Since we already know you can't count on your firearm to do it, it's ludicrous to expect a knife to do it.
Sure, a certain move or tactic might inflict a fatal wound, the difficulty is that it requires contact distance, and that alone guarantees you must be equally vulnerable to a countermove. Better to stand off ten feet and shoot. In fact, better to stand off 100 yards, even with a pistol. The entire point of weapons development in history is to create as much standoff room as possible to reduce injury to yourself.
From knives, to swords, to lances, to bows, the effort has been to keep expanding the standoff. We're up to cruise missiles and satellites, if you carry a gun, there is no advantage to stepping back to a lesser weapon such as a knife.
Carry it as a tool and don't be one.
I just changed up my EDC from a Benchmade Risk 950 to a Leatherman Sidekick, and honestly, it's done more work for me in the few days I've had it than the knife did in two months of carry. Would I carry the Risk in a high risk environment over the Leatherman, sure - but the primary weapon would be an 6.8SPC AR15 with a 9mm Glock as backup.
Knives are in a huge renaissance of designs and materials, but absolutely aren't any better as a weapon than they were 1,000 years ago. And a man armed with just a knife then was no match for one with a sword, lance, or bow.