iPhone 6s 4g LTE bands
 

I was under the impression that previously I couldn't buy an iPhone from the US and use it in Europe due to the 4g band differences.

However I've just read an article that says the 6s has about 23 different 4G bands, covering the US most of Europe and beyond.

http://www.cnet.com/news/can-a-globe-trotting-wireless-speed-demon-be-guaranteed-4g/

Just to make sure I'm understanding this correctly, is the iPhone 6s that is sold in the US identical now to the ones sold in Europe?

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.

This will show it all
http://www.apple.com/iphone/LTE/

Just to add to the comments above, I can confirm that our Verizon iPhone 6s (that's plural for the 6 not the 6s) do work in the UK. We just swap out the US sim cards for UK ones.

My wife's verizon 6s+ worked perfectly in Taiwan and HK with local post paid SIM cards