Why not both? Regular 1HKO would just put you on the bench for the remainder of the wave. And in 1HKO Extreme you're out for for the rest of the mission > ideal for those really balls to the wall batshit crazy mofo's, eh, I mean, really gifted players.

So I came up with some hard challenges for those that really want to test their skills:

1. Melee only challenge - Complete a mission using only melee weapons. No ranged lethal weapons of any kind allowed, including sentries, although non-lethal weapons (mediguns, milk, jarate) are fine.
2. Last stand challenge - Complete a mission by making a last stand at the hatch without leaving that area, except for money collection.
3. Hobo challenge - Complete a mission without a money collector. Accidentally picking up money while fighting is ok, but no deliberate collecting. Alternatively, if you're really hardcore, simply play without upgrading anything.
4. 1HKO challenge - Complete a mission without dying during a wave. Any player that does die during a wave has to sit out the remainder of the mission.