I've only heard bad things about AT&T from people trying to use iPhones while chasing. It's not even a debatable issue - once you get outside of populated areas, AT&T simply doesn't have the same infrastructure as any of the other providers. My T-Mobile phone also had a horrible connection anywhere but Minnesota, which like OK, can also have some dead spots for Verizon/Sprint.
I'd personally rank it as: Verizon/Sprint >> T-Mobile > AT&T, based on usually having one of every device in the car, and chasing in a box from WI to CO, and MN to TX.
GR really can't be beat by anything else out there at the moment. Sure, if you're just looking at reflectivity there are other options, but GR offers so much more. We use ThreatNet/WX Works primarily because it's a fairly cheap safety net and we already have the expensive hardware. I wouldn't recommend it to new chasers. It can be a very finicky system and we've almost missed storms because we were too wrapped up in trying to fix it when something broke.
A 3rd party level 3 feed (allisonhouse.com, etc.) is crucial. The public feed *will* be overwhelmed on MDT/HIGH risk days, and the $10/mo for a basic subscription is well worth it.
Also, the Cobra power inverters are amazingly reliable. Black and Decker inverters are junk.