Neither of those is really likely to instantly kill someone. Unless you slit a persons' throat or somehow get through the chest wall to the heart, which is hard, it will not be an instant kill. On the other hand, if you are shot in the chest with a 45mm round, you are at least going to fall down. I mean, if I stab whodatfever in the chest with a hunting knife, it is not likely to kill him instantly. He might not even be seriously injured. But if I shoot him in the chest with at 45mm round, he is likely going to die or at least be very seriously injured.
Regardless, it's supposed to be a first person shooter, not a first person stabber.