AMPA receptors mediate fast glutamatergic transmission. They've been studied for a long time, and their pharmacology is pretty clean. Metabotropic glutamate recepeptors are the GPCRs that glutamate binds to, and they are many, difficult to fiddle with pharmacologically, and difficult to understand because of the cascade of effects that ensue from glutamate binding to them, so I think it's the other way around