Gun owners like to say that guns don't kill, people do. Well, criminals willing to use violence without hesitation or compunction are well aware of that fact, and probably believe it on a more visceral level than the average law-abiding person or cop.
A problem with engaging in an intellectual discussion regarding how criminals may consider the "psychological v. physiological" distinctions involved in the use of a knife or a gun is that law-abiding people often have a hard time thinking like criminals. Habitual criminals don't seem to use the same reasoning in their actions as law-abiding persons.
You talk to enough criminals, you may get the feeling that they're more concerned about the person they're confronting than the specific type of weapon they may be facing. Meaning they're more concerned whether they're facing another criminal, than a law-abiding person (victim) or a cop.
Habitual criminals know other criminals aren't concerned about breaking the law and will use violence without warning or compunction, while regular people are afraid of going to jail (losing their jobs, etc) and cops have to act within the rules (laws and policies) or lose their jobs.
This was one of the things mentioned in the LEOKA class I attended (a federally sponsored training program of tracking Law Enforcement Officers Killed and Assaulted), and which was developed after a lot of interviews with convicted suspects of violent crimes. It was unsurprising and was similar to what I'd come to learn after a career's worth of facing, arresting, talking and interacting with folks who spent a lot of time being arrested and convicted of crimes.
The people who accept that they may live or die by violence on any given day (criminals) seem more concerned by the people they may face. Guns and knives don't possess the will to be used, but the people who wield them may ... or may not.
Knives (or other cutting instruments) of any sort of pretty ubiquitous and easy to come by in our society. Just visit the nearest hardware store, flea market, etc. Guns are a little more difficult for criminals to find, but theft keeps them pretty available.
Just some thoughts.