How is Left4Dead versus mode different than Gears of Wars 2 Horde mode?

Kotaku tells us how...

Nothing like more Left 4 Dead coverage to wipe that Gears of War 2 taste out of our mouths. Kotaku has an article up called, accurately enough, Why Left4Dead Versus Isn’t Horde Mode.

It's a lengthy read but goes in-depth into the Versus mode of Left 4 Dead. I know, I know, we only really care about the co-op mode but it's nice to know that other modes exist.