I tested extensive both. Finally I spent more money and went for the AT. MB has the best automatics available. And with a little training period you can control the car perfectly by using the Autostick! This is my first AT car as AT isn't (or was not) very common in Europe. But I'm wholly happy and enjoy the choice between D and Autostick.
The manual shifting is cool working, but the clutch way is definitely to long.