There are three versions of the iPhone 6S. The A1633 and the A1688 are basically identical except that the A1633 has LTE band 30, which is used by AT&T in high density locations. Other than that, they contain all the LTE bands you would need for use in every country, including European ones. The other version is the A1700, which is basically made for Chinese markets.

http://www.apple.com/iphone/LTE/

Historically there have been bigger differences between the radios in each iPhone model, but the last couple of years the US ones have been fully LTE compatible with EU networks so as long as they are unlocked.

I recommend buying the SIM-Free version directly from Apple. For all intents and purposes, it will work everywhere.