Currently, Apple claims they are not selling an 'Unlocked' version of the iphone 6 but I would guess that the TMO version is going to work. I think this version is actually unlocked but Apple calls it the TMO version because it comes with a TMO SIM card in it.
My iPhone 5 from ATT does work on the Straight Talk network because in my region ST is merely reselling the ATT network. Additionally, last month Apple needed to replace my iPhone 5 for a manufacturer defect and they gave me a TMO iPhone 5 as the replacement. This phone is definitely unlocked, as I've used it not only on the ST/ATT network here in the US but on TIM's network with a TIM SIM card in Italy and on Vodafone's networks with Vodafone SIM cards in both Portugal and Hungary.
In some regions, ST is reselling the VZW network so I would assume in those regions the VZW iPhone 6 is going to work out of the box but again I think the TMO phone should work regardless of the network as it has all frequencies built in.