Both are important. You can argue in circles about the relative importance.

The wheel is a platform innovation, the value of which lies mostly in its conceptual existence which enables useful applications of it. No wheel, no useful applications of the wheel.

The steam engine is another platform innovation. By itself, all a stream engine does is is sit there, eat resources, and belch white smoke. Combine it with some wheels and a large cart. Now you have a game-changing transportation method.