The blue keg is a ball lock, also called a Pepsi keg. The green one is a pin lock, also called a Coke keg. Those are the two big companies that chose each style of course. Your black QD, quick disconnect is a liquid ball lock. The gas QDs are gray. Gray=gas black=beer. The keg should be labeled in/out and some have a dimple or a cut in the handle part next to the gas post. I would guess the dimple or cut is to figure our what post it is under bad lighting conditions.
There is no reason not to have both but will require more connections. The ball lock kegs are also a little teller and thinner than the pin locks so that might be a consideration.