For the user, in the near future, it shouldn't make any difference. Where you now connect to a Sprint tower, you'll continue to connect to a Sprint tower, where you now connect to a TMO tower, you'll continue to connect to a TMO tower.
But I suspect (due to prior experience - see Nextel - and Legere's statement that it will take at least 3 years to "migrate Spprint's customers to TMobile") that Sprint will eventually go away. Then we'll have 2 GSM carriers - AT&T and TMobile - and one CDMA carrier - Verizon. (And, supposedly, TMO's prices will decrease - we'll have to see about that.)