Both use GSM and similar bands so it shouldn’t take much to integrate the two networks. It isn't just about networks anyways it is also about all important spectrum. Typically when a network is getting overloaded it is because of a lack of spectrum to build more network into not an unwillingness or in ability to build more network. These guys have really deep pockets and it won't take long from them to make use of any under used spectrum from the other network.

Everyone will be moved to AT&T but so will all T-Mobiles towers, radios and spectrum. In other words the network itself will be moved into AT&T. Towers are another issue as well. Providers spend a lot of time and money trying to secure towers in the right location. Most cities won’t allow them to build new towers so they have to make do and co-locate their towers (when two companies have their antenna very close to each other). They should be able to improve their network simply by moving their radios to whichever one has the best location. It will also improve the network by cutting down on co-located towers which can interfere with each other.

From a technology standpoint this will be great for AT&T customers who are in heavily congested areas and it should also help most T-Mobile customers by giving them a bigger network with less dead spots and dropped calls. How much it helps a specific user will depend a lot on where they live. Here in Dallas the AT&T network is fine since it is such a spread out city but T-Mobile has patchy coverage. So AT&T customer won’t see much difference but T-Mobile users will be better off. In NY where the AT&T network is heavily overloaded AT&T customers will see improvement but T-Mobile customers will take a hit.