This interactive maps shows T-Mobile and Sprint coverage after the two companies complete a merger (via Esri).
Merged Coverage Map
Note that the map is not final, and is meant to be more of an estimation. You can click and drag around the map, or type in your zip code for faster results. On the left side it shows cellular coverage pre-merger, while the right shows post-merger coverage. After the merger the remaining company will keep the familiar T-Mobile name.
You can view the map here.