horsepower ratings are mostly for marketing purposes and bragging rights (for racing purposes, it is a different matter, though)
what matters is usable power. By "usable", we mean at the rpms we use on a daily basis. This is determined more by the shape of the torque curve, and the torque peak itself. The higher the torque peak and the flatter the torque curve at the lower rpms, the more usable the engine power.
That's why those large trucks, buses and payloaders only have only so much horsepower, but have gobs of torque at what is practically idle speed to us car drivers - and most of that power is put to use.
the torque curve is flattened by the use of features like variable-length intake manifold systems and variable cam systems (also forced induction), which modern benz engines have. Probably the twin-spark feature also contributes to this.
horsepower is proportional to the torque and the rpm. Some cars like the Honda S2000 extract a lot of power from small engines but you have to rev the engine past 7000 or 8000rpm to do this. Not very usable in everyday driving. And you will likely spin your tires in the process.
so the bottomline is torque: speak softly and carry a big stick ...