Something that we don't talk about very much is that all Verizon iPhone 6 and 6 Plus are going to be unlocked, even if they are branded.
You can put an AT&T or T Mobile nanoSIM into a Verizon iPhone 6 and you will be just fine.
If you've bought an unlocked "GSM" iPhone 6 (the ones that come with a T Mobile SIM), you will be able to use both AT&T and Verizon SIM in them.
In that case (a Verizon SIM in a T Mobile iPhone) the only problem you'll have is if you are in a Verizon coverage area that is *not* LTE yet, because the unlocked "GSM" iPhone might not have the CDMA bands activated.
So a Verizon phone will have all of the GSM bands activated and will work with all US carriers in all areas. A T Mobile phone will work on all carriers in all areas except for a Verizon area in which there is no LTE.