Unless you leave California often coverage shouldn't be a problem. I'd say make your decision based on phones/price/customer service.
Cingular's the only provider I've had extensive experience with. They have good family plans, but their customer service is ass and they aren't too reliable. I get a lot of "Network Busy" errors with them here in Seattle, and I average about 1 dropped call for every 10-15 hours of talk time (signal strength randomly drops to zero, but then picks back up). The only reason I stayed with Cingular is because it's what all my family has, so the mobile-to-mobile is nice. The only reason my family has it is because a lot of my mom's side work for AT&T, so get discounts.
I recently went snowboarding at Steven's Pass and I lost service completely, while the girl I went with (serviced by Verizon) had zero bars but wasn't out of network, and was still able to recieve incoming calls (although they dropped often). The info desk attendent said T-Mobile was the only provider with a tower near-by. I don't know if things like that are important to you, but they are to me. When I'm in the middle of nowhere, especially in a dangerous environment like a mountain, I feel a lot more secure knowing I have a working phone. If you or your family have any hobbies that involve extended periods of time in not-so-populated areas, you might want to check coverage maps to make sure you won't be boned when you need your phone most.