Right now in the U.S., you're stuck with AT&T if you have an iPhone, or either AT&T or T-Mobile if you have a Windows 7 phone. That will change in 2011, but at the moment,
you don't have another choice if AT&T doesn't have coverage in your area or costs more than you want to pay.
With Google, on the other hand, you can pick the best carrier in your area for your budget. Each one will have at least a few great Android phones available.