I think T-Mobile has a point with 5G on 600Mhz which is expect to double its relatively slow speeds. Sprint is expected a 5 fold increase in Band 41, however I personally think the real difference will be in upload speeds given 100Mhz will be available for upload as compared to today's typical 20Mhz that most Sprint customer observe. T-Mobile's plan given merger approval is to use the spectrum to shift customers around while it upgrades each band to 5NR starting with Band 41 (doing this on a market by market basis). This is according to publicly available plans and comments. Since those were released the plans have been revised.
My guess is the FCC is pushing T-Mobile - Sprint towards a Shentel - nTelos type of deal where coverage must remain the same, customers must end up with a phone in some fashion that fully supports the new network, some divesting of spectrum, and they must report back to the FCC on progress towards these goals. The only area that is really new is the MVNO possible conditions. To placate possible opposition and appeal to FCC stated goals, wireless internet is thrown in, at least for rural areas, which is also a shot across the bow for cable companies getting into wireless. Doing 5G earlier would allow the new T-Mobile a longer time frame to depreciate these assets. It is also expected that new "must have" applications will be developed as a result of the increased speeds.
In Champaign, IL, AT&T has the densest network although they have been really sloppy about getting all of their spectrum on air on every site. Some sites are L700/L800/L1900, others have AWS and/or WCS LTE live too but lack CLR LTE, etc.
Overall AT&T’s network reliability is second to probably only Sprint for it seems the equipment AT&T is using has considerably poorer range/propagation than the equipment Sprint is using. When broadcasted from the same location at the same elevation and angle, AT&T L700 drops out when Sprint L800 is at -112 to -114 RSRP. That being said, AT&T devices do hold onto LTE for dear life when it is available. Their qRxLevMin on lowband LTE is set to -124 dBm, and if I ever drop to WCDMA/HSPA, my iPhone always jumps back to LTE in 15-20 seconds.
T-Mobile lacks lowband spectrum in the Champaign market which immediately disqualifies them (for indoor coverage is challenging) and Verizon’s site density is awful.