Also note that AT&T has 10MHz LTE channels only in 1/3 of the country. In another third, they have 5MHz channels. In the last third, they have zero 700MHz spectrum and will need to deploy LTE in another band (which won't be any time soon and will likely be 3MHz channels, if anything).
So in their 10MHz markets, they can reach 50Mbps speeds...but it won't be for long. In their 5MHz markets, they will likely be slower than Sprint, because they have less site density. And in 0MHz markets, it's safe to say Sprint will kick their butts.
Robert