I think the version of the iPhone 6 that will work with Sprint won't be that dependent on the config for AT&T or Verizon. If you look at the details of the model numbers for the 5s, the other big three carriers are using the same model number:
iPhone 5s (Model A1533) is used by AT&T, T-Mobile, Verizon (and 8 or so other small carriers in the U.S.)
iPhone 5s (Model A1453) is used by Sprint, US Cellular, and a bunch of the CCA members in the US. SoftBank is also on the list of carriers for that model number.
I know some folks that have purchased a Verizon Wireless iPhone 5s and then successfully used it on AT&T and T-Mobile's networks (including full support of the LTE & GSM bands.)
http://www.apple.com/iphone/LTE/
I also thought that AT&T agreed to support band 12:
http://www.androidcentral.com/att-will-commit-700mhz-lte-network-interoperability
http://s4gru.com/index.php?/topic/4643-att-acquiesces-says-goodbye-to-band-17/