AT&T mobile will be distinctly inferior to Verizon for years. The practical difference will likely be small, but AT&T is gambling people won't choose Verizon. While a handful of hotspots will go mmWave, AT&T's Andrew Fuetsch said mmWave will only be used in hotspots. Most of the "5G" network will be low and mid-band.
Over strong objections of their technical staff, AT&T has decided not to build millimeter wave 5G widely. Instead, they will use lower spectrum bands with 70% to 90% less capacity.
Almost certainly, the United States will now have only one high-performance 5G network; almost everything else will be 4G LTE with a software tweak and a massive publicity campaign. Every engineer building networks knew this was hogwash. MWC in Barcelona was dominated by emperors without clothes. See.
What seemed like silly semantics and a cheap pr campaign is now proving to have real-world consequences.