I switched to AT&T from T-Mobile this month just to have extra coverage when travelling to some mountains in Nevada, but my experience has been the following when using it day-to-day in South Florida:

AT&T Pros:

-Better in-building coverage...even in buildings where I got a Band 12 signal with T-Mobile

-Greater coverage in rural areas

• You can always switch back to T-Mobile

AT&T Cons:

-Slower speeds than T-Mobile..less consistency

-If you are using an unlocked phone that doesn't support AT&T's carrier aggregation, your phone will be switching back and forth between AT&T's four LTE carriers here in South Florida (5x5 Band 4, 5x5 Band 2, 10x10 Band 5 and 10x10 Band 17). This means that it will sometimes hold on to a weak Band 4 or Band 2 signal indoors instead of switching to one of the lower-frequency bands...and this means that speeds are not as optimal as when on T-Mobile's solid 20x20 Band 4 AWS carrier since they will vary depending on load and the fact that two carriers are only 5x5 means they will not be ideal either.

-Call quality is inferior to T-Mobile

-No DC-HSPA fallback...I mean HSPA on AT&T is decent here (Band 5) but T-Mobile's HSPA here blows it away.

-You have to visit a corporate AT&T store or call them to port your number to GoPhone..you can't do it online like with T-Mobile

T-Mobile Pros:

-Faster speeds

-Music freedom

-A solid 20x20 LTE signal instead of four different fragmented LTE carriers, which helps give you consistent, faster speeds

-DC-HSPA fallback sometimes rivals its LTE (at least here in South Florida where T-Mobile has excellent spectrum holdings)

-Band 12 signal greatly helps indoor coverage

T-Mobile Cons:

-Band 12 signal does not reach indoors everywhere, so you still get fallback to HSPA or EDGE..but then again..AT&T's network is not perfect either and also loses signal indoors...low-band frequency helps but is not a silver bullet for every coverage gap. Band 12 can also be slow at times

-Prepaid plans on T-Mobile proper are not as competitively-priced anymore, but there's always MetroPCS

-Not enough hours in a day nor excuses to leave my house and it's Wi-Fi to enjoy it's super fast network.