If that knife fits the bill as a utility weapon than a jury just might buy it. But then again if you've got a gun it's not likely you'd use the knife for self defense. Your personal weapons carry choices are irrelevant and we're dealing with a lot of hypotheticals here. Again I'm not talking about a clear case of self defense, I'm referring to the gray areas. Look at the case of the atienza Kali guy (Umalii believe was his name) in the trial it was noted that he had a specialized knife and it worked against him. Did it seal his fate? Maybe not, but it was a contributing factor to his imprisonment. My point and I think the point of the op is that your choice of weapon could be damning in a court case, especially if you're choosing to carry said weapon as a less than lethal option.
Obviously laws vary widely from country to country, but in the US (which is where I live) the weapon is irrelevant. You're either justified in killing them, or you're not.
And although I am not a lawyer, I have spoken with a number of lawyers and police officers about this issue, and the consensus is clear.
If you use a weapon, don't BS about it. State clearly that you used a weapon because you were in fear for your life and that you intended to shoot/stab/bludgeon them.
In criminal court, you're either justified or not. But if you say something along the lines of 'I didn't mean to shoot him" then you're opening yourself up to civil charges for negligence.
As for using the knife as a weapon... it depends on the specifics. Am I on my back with my gun under me and an attacker on top? I might well go for the knife, depending on other factors.