At home T-Mobile only gives me 2 bars 3G (although their coverage map says I should be getting LTE). In my half below ground garage there is no change in signal.
I was on AT&T's unlimited plan and finally ditched them for T-Mobile because they kept restricting services (no pairing, no wifi) and raising prices. Love T-Mobile.
The latest Consumer reports ratings from their website has Consumer Cellular (which uses the ATT&T network) at 89, T-Mobile at 76, Verizon at 72, Sprint and Verizon tied at 70. Data collected as of Summer 2016. A quick look at at City By City listing shows T-Mobile better than Verizon in a lot of cities. However Verizon is rated better than T-Mobile in most of the summary data categories (voice, text, web) other than data. Consumer Cellular is rated better than all of the 3 major carriers in all categories, which is a little puzzling since they use AT&T's network. As the ratings are somewhat confusing, I would recommend that you read them yourself.
I just completed a 7000 mile road trip through 14 states and had no troubles with T-Mobile until I hit Montana, where I didn't have any coverage at all. Well, actually my car didn't have coverage. It has just 3G on a separate T-Mobile account. I didn't check my phone so LTE coverage was likely there; my car just didn't see it. Their overseas capabilities are unmatched (unlimited 3G data, 20 cent a minute calls).
As you can read in various MacRumours forums T-Mobile leads the industry in innovation and pricing. The other carriers are forced to respond. I think Verizon now has a data cache, but it is only for one month rather than 12. And unlimited data is back for a cheaper price than the other carriers (when you can even get it) with T-Mobile One, albeit with the >~25 MB slowdown.
My monthly bill for my Simple Choice line is:
$50 for one line with 2 GB of data
$10 upgrade to 3 GB of Data
$5.83 for taxes and fees
$65.83 total
$10 for iPad 5 GB of data
$75.83 total.
My current Data Stash is 16.89 GB.
If I went to T-Mobile one my charge would be
$75 for the phone - unlimited everything
$10 for iPad 5 GB data (if I didn't want to use teathering). Unlimited data for iPad is $75.
$85 Total
It all boils down to whether in the locations where you use cellular whether or not the carrier you choose works. T-Mobile offers a 30 day tryout. If their coverage doesn't meet your needs then you can "return your device for a full refund of service and device related costs".