In my opinion, you need both the sim and the teacher as Da Rock said. While the sim cant teach you setting up your airplane, it will teach you muscle memory. That will help you react faster when in a tight spot. Provided, of-course, you take the sim seriously.

I do disagree that the sim cant teach you to fly an underpowered airplane. In fact I learnt that way. You dont need to setup your airplane on the sim to be underpowered. Just try to do all your flying at 40% power on the sim. Such a small excess of power will have the same effect as an underpowered airplane.

Of course, this is just my opinion.

Ameyam