Well, it mostly depends on where you live. In most metro places, coverage is relatively the same. Here are just some of the advantages for each:
While both are not great for rural coverage, ATT would tend to have more than T-Mobile. Also, there are some places where T-Mobile still has EDGE (basically 2G), where ATT has HSPA (3G). Overall, ATT has a little more voice coverage than T-Mobile. However, due to capacity issues, some ATT users have complained about dropped calls. And of course, ATT has the Iphone, while T-Mobile does not.
Tmo is the only major carrier that has WiFi calling in its phones. Pretty much all of its newer Android devices have it, as well as Blackberries. WiFi calling is something that... (continues)
I have to commment on this, wifi calling actually does use your minutes, It gives you a message everytime you open up the wifi calling and informs you that wifi calling uses plan minutes. Of course Tmo to Tmo there is no charge or nights and weekends but your daytime minutes are still used while using wifi calling.
You also forgot Tmo Data Pricing in that, Tmo offers 2gb data for $20.00 while AT&T offers 3GB for $30.00. however with at&t if you go over the data it is $10.00/GB overage and Tmo just slows your speeds.