Apple just made this decision tough. The AT&T and T-Mobile versions don't support Verizon anymore even if they are unlocked, because they lack CDMA. The Verizon and Sprint versions, if unlocked, do have all the bands that AT&T and T-Mobile need, including bands 30 and 12. I would assume that the SIM-Free model should support Verizon and Sprint as well, but who knows when it will be available.
I guess the best model to get on launch day is the Verizon model this year, but only Verizon users are able to pre-order from the Apple website. Everyone else will have to try to buy it device-only at a local Apple Store.
Yes, I too was disappointed to see that the AT&T and T-Mobile versions have dropped the CDMA bands.
I don't actually have "real" Verizon - I'm using Verizon through Tracfone's BYOP program which allows me to get on Verizon's network without paying as much (some limitations, like no VoLTE). I'm using an iPhone 6 that I originally had on contract via AT&T and then after I got it unlocked, I use a Tracfone Verizon sim and it works great; Verizon LTE speeds and coverage are great where I live.
I guess that means Sept 9th purchase for me; I will have to wait for the "SIM-free" version and hope it comes with CDMA bands (they wouldn't disable it on that, would they?!)
Yep, kind of sucks that Apple disable the bands for no reason... especially when we are buying them for full price

(probably was the ****ing carriers that asked Apple to do this so when people wanted to switch carriers they would be forced to buy "their brand" of iPhone 7)...
http://www.apple.com/iphone-7/specs/
Model A1660*
Model A1661*
FDD-LTE (Bands 1, 2, 3, 4, 5, 7, 8, 12, 13, 17, 18, 19, 20, 25, 26, 27, 28, 29, 30)
TD-LTE (Bands 38, 39, 40, 41)
TD-SCDMA 1900 (F), 2000 (A)
CDMA EV-DO Rev. A (800, 1900, 2100 MHz)
UMTS/HSPA+/DC-HSDPA (850, 900, 1700/2100, 1900, 2100 MHz)
GSM/EDGE (850, 900, 1800, 1900 MHz)
Model A1778*
Model A1784*
Models A1778 and A1784 do not support CDMA networks, such as those used by Verizon and Sprint.
FDD-LTE (Bands 1, 2, 3, 4, 5, 7, 8, 12, 13, 17, 18, 19, 20, 25, 26, 27, 28, 29, 30)
TD-LTE (Bands 38, 39, 40, 41)
UMTS/HSPA+/DC-HSDPA (850, 900, 1700/2100, 1900, 2100 MHz)
GSM/EDGE (850, 900, 1800, 1900 MHz)