Better? I take it you mean which ones are easier on us, the pray? Then I'd say the Call of Duty ones.
They are easier to kill, and we still stand a chance against them, even with old World War II weapons. While the ones in the Left 4 Dead series have man mutations, and variants. Which would mean - faster death.
I'd say going against three zeds from COD is better then going against one tank from L4D no?