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:

ATT-

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.

T-Mobile-

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 is free, doesn't use minutes, and allows a person to have cell phone coverage from a nearby router, even if no cellular tower is nearby.

Also, T-Mobile has enhanced its HSPA network to HSPA+, that deliver speeds up to 42mbps. While not quite as fast as LTE, these speeds are very fast, providing a 4G like experience. T-Mobile has this enhancement in the majority of its markets. ATT also uses HSPA+, but has speeds only up to 14.4mbps. Perhaps ATT has some towers at 21mbps (I don't really know); however, it does not have the devices to support it. That being said, ATT does have limited markets that use LTE 4G, while T-Mobile has none.

Other than that, you can compare plan prices, in which you can clearly see the differences. I hope this answers your question.