In general team-building theory (not in any specific team case), I fully agree with Wave that a top-flight quarterback is always more important than a top-flight receiver, but that goes for every single position on the field. The quarterback is always the most important piece.
To the Broncos specifically, I just look at cap estimates and I don't see why Denver can't have both, and in addition have faith in the front office that they can make that work under the cap restraints, just like they did with Manning. I'm not convinced Cousins is that guy, but the Broncos might go that route because he's the best option.