Well I'm no top racer or tuner so if I'm wrong someone please correct me...

The way I look at tire vs spring is that tires are usually a more radical change and springs/shock position is more fine tuning. If I need a large change in handling I look to tires but if I'm close I look at setup. Also I try to take in account what other people are running and if I am running those tires already I would be more inclined to think it was my setup.