I think you get xp for knocking people out, and then extra xp for actually killing them. The vids seem to suggest that you do. I have read that you gain a lot of xp for melee-striking a knocked out opponent, and almost nothing for shooting them when they are down.
I think that they don't award xp for either of those things, they just award xp for damage, that way, players can still get rewarded just as much as the finishing guys, even if they're softening lots of enemies up and leaving their teammates to finish them.
Also please don't be too hard on this thread starter guys, not everyone can be bothered to go through seas of text and watch all the videos, and considering SD is trying to make the game accsessible with Challenges, ranked servers and lots of on-screen hints, let's not be hostile and ruin it.