OK, OK!
Sheesh, chillax.
I think they are each equally imprtant. Mapping is just the environment, and you need models to live in that environment to make the experience more life-like or involving. There are many kinds of models and many styles of mapping.
Now, here's where it sways. A Modeler is the mapper's bitch if the mapper needs props and such. But the character modeler gets paid more than the mapper because he makes the one component you see the most and he's paid a fat-load to sell it. The mapper makes something thats a little less frequent. Sometimes there are areas in a game you see only once. Now, mappers are most likely to be promoted to LEAD GAME DESIGNER because they DO have an imagination. But not only that, they fully understand how everything reacts with what and where. Good mappers evolve into designers real fast in the industry.
Mappers start small, then pimp all.
EDIT: NO, mappers DO NOT DO IT ALL BY THEMSELVES.
You have a layout designer, concept artist, primitves, detailer, texture artist, lighting, and a level programmer.