I'm going with the T-Mobile w/o contract
Since there is conflicting information from previous user experiences and what you hear from Apple, I assume from what I've read here and looking at the various sites the worst case is that the T-Mobile phone will have to be on their network for 40 days (or $100). If I'm lucky it will be unlocked. Unless we get an definitive statement from T-Mobile or Apple we're just not going to really know by the time we pre-order tonight.
A T-Mobile phone is nice as they have ridiculously low international data and call rates (comparatively). When I travel overseas I take out a $50 T-Mobile plan, put their sim in my unlocked AT&T phone, and then cancel the plan when I return from the trip.
Ideally I would like (ironically) the Verizon CDMA phone (Model A1586
or Model A1524 I think) as it supports the most frequencies: GSM and CDMA. (see below) which is critical when using it internationally. My guess is that there will so much initial demand that Apple wants to meet the needs of its biggest customers, Verizon and AT&T in the U.S. AT&T and Verizon want people tied into contracts, so Verizon and AT&T unlocked phones will not be available until some date in the future when supply and demand become balanced.
Once I can get the T-Moble 6+ phone unlocked (on arrival or 40 days later), I'm planning to ditch my AT&T grandfathered unlimited plan. Over it's lifetime (whatever that is) I've used 2 hours of phone and less then 5 GB of data so I'm throwing my money away.
I'll then go with Consumer Cellular. They use AT&T's network. Top rated by Consumer Reports. I can get a limited call and data plan for <$20. And if I'm traveling in the U.S. (overseas I'll get a temporary T-Mobile plan) if I go over call or data limits they text you. You can then immediately upgrade to the plan you need, and downgrade when you return home. And by buying the phone I can upgrade every year, and the phone I'll be selling will be relatively current so ebay pricing shouldn't be too bad.
Here are the frequencies that the various phones support. I'm reading this that A1549 and A1522 are the AT&T and T-Mobile phones, don't know which, and that their specs are the same:
Apple Site
http://www.apple.com/iphone-6/specs/
Model A1549 (GSM)*
Model A1522 (GSM)*
UMTS/HSPA+/DC-HSDPA (850, 900, 1700/2100, 1900, 2100 MHz)
GSM/EDGE (850, 900, 1800, 1900 MHz)
LTE (Bands 1, 2, 3, 4, 5, 7, 8, 13, 17, 18, 19, 20, 25, 26, 28, 29)
Model A1549 (CDMA)*
Model A1522 (CDMA)*
CDMA EV-DO Rev. A and Rev. B (800, 1700/2100, 1900, 2100 MHz)
UMTS/HSPA+/DC-HSDPA (850, 900, 1700/2100, 1900, 2100 MHz)
GSM/EDGE (850, 900, 1800, 1900 MHz)
LTE (Bands 1, 2, 3, 4, 5, 7, 8, 13, 17, 18, 19, 20, 25, 26, 28, 29)
Model A1586*
Model A1524*
CDMA EV-DO Rev. A and Rev. B (800, 1700/2100, 1900, 2100 MHz)
UMTS/HSPA+/DC-HSDPA (850, 900, 1700/2100, 1900, 2100 MHz)
TD-SCDMA 1900 (F), 2000 (A)
GSM/EDGE (850, 900, 1800, 1900 MHz)
FDD-LTE (Bands 1, 2, 3, 4, 5, 7, 8, 13, 17, 18, 19, 20, 25, 26, 28, 29)
TD-LTE (Bands 38, 39, 40, 41)
All models
802.11a/b/g/n/ac Wi‑Fi
Bluetooth 4.0 wireless technology
NFC