If two weapons have the same stopping capability, you must always pick the less-lethal one.
From an immediate humanitarian standpoint of preserving lives, I can see what you mean. However:
I completely disagree. Stopping capability is directly correlated to lethality, I'm sure you find this reasonable? Not only is a more lethal weapon more capable of stopping the offender, its more likely to preserve the life of the defender, and more likely to result in the chances of a non-lethal confrontation by nature of scaring off the offender. If the offender finds himself at gunpoint, I think it reasonable to say he will fear for his life and be more likely to attempt escape without violent confrontation, whereas a person defending himself with anything from a tazer to a baseball bat is a target much more susceptible to being killed by the offender.
On the flipside, a confrontation between two less-lethal weapons may result in more outcomes of violent confrontation without lethality, but also less outcomes of non-violent confrontation.
In both cases, there is simply too much left to chance: a swing of a baseball bat to a temple can be just as lethal as a gunshot or knife slash or a katana/severed hand. The defending party must be allowed to defend himself with whatever means is most readily available, and whatever is most likely to affect his safety, even if that should mean using a weapon with a larger degree of lethality. If he is restricted in his means to defend himself, that allows the offender greater capability to victimize the defender, and it should be the rights of the defender we're worried about, as he is the one in good standing with the law at this point.
I agree there are cases where the rights of the offender can be taken to far; he should not be allowed to empty clips and clips of bullets into an offender or ruthlessly slaughter him, but the use of lethal force as a defense must be allowed.
The use of lethal force would also act as a deterrent to crime, if not by scaring people out of committing crime, at least by removing or crippling criminals. I don't know how well it would 'scare' people out of committing crimes, as that's already as risk, but I think if every person in America owned a weapon there might be a noticable deterrent effect. Possibly it could be accompanied by an increase in murders/deaths for a while, but in the long run I think it would help deter household crimes at least.
None.