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Is a major metro area so either verizon or ATT should be fine but you really want to check out the coverage map in the chance that at either your house or work happens to have a weak tower coverage.
 
AT&T used to be notoriously bad in San Francisco, however they have improved coverage recently. Down in Silicon Valley, both Verizon and AT&T should provide the best coverage.

As far as I can tell, my buddies using Verizon seem to complain the least.

I've used AT&T and T-Mobile (both prepaid and MVNOs), and T-Mobile's breadth of coverage is definitely inferior, even in heavily populated urban zones, often dropping to "4G" (HSPA+) or even EDGE (2G) in certain neighborhoods. This is probably not identifiable on a coverage map.

T-Mobile is even worse when you get to sparsely populated areas (like the Santa Cruz Mountains or wine country).

The biggest complainers of all around here are Sprint customers.

I'm currently using a prepaid T-Mobile SIM in my iPhone, works well enough for my purposes.
 
I've used AT&T and T-Mobile (both prepaid and MVNOs), and T-Mobile's breadth of coverage is definitely inferior, even in heavily populated urban zones, often dropping to "4G" (HSPA+) or even EDGE (2G) in certain neighborhoods. This is probably not identifiable on a coverage map.

Yeah, I automatically think voice when people ask question like that, but sure, I wish they have a good map for 4G/LTE.

Coincidentally, Walt Mossberg just had a recent 4G/LTE "test" for 5 metros, and one is Silicon Valley (he shows a picture of the Apple campus), and he gave AT&T as #1, Verizon #2. Probably didn't go to ALL neighborhoods. ;)

Back to voice, 10 years ago when I had Verizon, I don't remember I ever had any call probs. Now I am with AT&T, work is strong, but home I only get 2-3 Bars. :(
 
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