What would I use? Realistically, guns, even at close range. Fuck your pretenses of honor, if someone else is trying to end me, I prefer to kill the other guy as efficiently as possible. It baffles me that people can list all the drawbacks of a firearm, but seem completely oblivious to the fact that the sword is no longer used in modern conflicts because the firearm made it obsolete in every respect, and not just because conflicts are fought at greater distances. Don't try to pull circumstantial bullshit on me, like who's trained in what, because you shouldn't have a weapon you aren't proficient with, and if you have no choice, you obviously should have done more to avoid the situation. A firearm will always be a more useful companion in an engagement than a sword, no matter what range it takes place in. Life isn't like Halo where the guy with the plasma sword can dice the guy with the battle rifle when he's close enough; you don't have a shield to absorb the rounds that guy almost always soaks up in the process of engaging an aware target (if you can sneak up on your target, it doesn't matter what the fuck you're using, so that's a moot point), and furthermore, a gun does not stop being a viable weapon just because you're standing too close to me, because the principle of a gun remains the same: all I have to do is make sure the noisy end is pointed at some part of you when I'm pulling the trigger.
Don't kid yourself: nobody shrugs off bullet wounds like they do in the movies, otherwise we wouldn't be using the ineffectual pieces of crap. If my bullet connects with you, it's going to hurt like a son of a ***** unless it kills you instantly or causes severe neurological trauma that renders you incapable of feeling localized pain.
Real life isn't Devil May Cry, either, where bullets chip away at an enemy's health until you're close enough to slice them up. I cannot stress enough how lethal a modern projectile weapon is, else the Marine Corps would use those swords for more than just ceremony.
A real-life combat scenario isn't going to accommodate your romantic fantasies of using a forgotten weapon to stop a modern evil-doer. Your overly-specific idealistic encounter with a bad guy, where you simultaneously get the drop on him and manage to strike a debilitating area with minimal training, is not going to happen in this universe; even if it did, if just one of the factors working in your favor wasn't present, you'd still die. Also, keep in mind that life isn't a video game where you're presented a fair and ample opportunity to get a lucky strike in: the other guy wants you dead just as badly, and is just as capable of killing as you are of killing him (unless he has a gun, in which case, you'd better hope he's too intimidated by your renaissance faire blade to point his weapon at you). Something else to keep in mind is that video games are not even close to being an accurate portrayal of firearms, because if they performed realistically instead of being balanced to accommodate different kinds of players, one well-aimed torso shot with any gun of sufficient caliber would be debilitating, and would only require a follow-up shot to put the poor bastard out of his writhing agony.
Sorry to be a buzzkill, but I really don't have the patience for people who dismiss modern weapons as ineffectual and make clear their intentions to get themselves killed in a truly stupid attempt to play samurai. Against a guy with a sword, I would use a gun. Against a guy with a gun, I would also use a gun. If you would choose differently, I'm afraid your understanding of combat is laughable.