A polygon is a triangle(well, not always but almost as far as gaming is concerned) that is used to build a 3D world.
A pixel is one "dot" on your screen. You can't have polygons without pixels so it isn't a matter of being better, for 3D games they need each other right now.
The numbers you are talking about are fillrate(ability to push lots of pixels) versus geometry(lots of polygons). If you are comparing an XBox to a GeForce3, the XBox is better. It uses a modified(enhanced) GeForce3 chip which is also being made by nVidia.