Purchasing an iPhone 6 in the United States - Using it in the United Kingdom I would like to purchase the new iPhone 6, but as I travel a lot between the US and the UK I have to make sure I buy a model that works well in both places. The idea is to buy an unlocked device so I could switch between American and British SIM cards. 

I have come across a few articles that state there could be issues with connectivity as the model of the phones sold in US differ from those in the UK. A few of the articles are listed here.


MacWorld (read the second section of the article). 
Reddit  
Reddit


In essence, they say that the main issue is bandwidth. Different models tune into different bandwidths and not all carriers support the same bandwidth. 

This page on the Apple website shows the different models, the bandwidths that they are suited for, and the carriers and the countries that support them. It says that the model A1586 (GSM) and model A1586 (CDMA) support the following LTE bands


1 (2100 MHz)
2 (1900 MHz)
3 (1800 MHz)
4 (AWS)
5 (850 MHz)
7 (2600 MHz)
8 (900 MHz)
13 (700c MHz)
17 (700b MHz)
18 (800 MHz)
19 (800 MHz)
20 (800 DD)
25 (1900 MHz)
26 (800 MHz)
28 (700 APT MHz)
29 (700 de MHz)
38 (TD 2600)
39 (TD 1900)
40 (TD 2300)
41 (TD 2500)


They also say the these bands are supported by the following UK carriers 


3
EE
O2 (including Giffgaff)
Vodafone


Here we see the specification of the iPhone 6 model sold in the UK which says the following LTE bands are supported:

LTE (Bands 1, 2, 3, 4, 5, 8, 13, 17, 19, 20, 25)

Which brings me to 2 questions:


If I buy a model A1586 (GSM) or model A1586 (CDMA) will I have good
connectivity in both the US and the UK?
Is there a difference between the GSM and CDMA model?
You must buy the GSM model if you plan to use it in UK. There you could only buy this model because all carriers across Europe using GSM technology. If you visit UK Apple Store, you can only buy that model. Check Tech Specs.

If you are on US, you need a sim card from a carrier that use a GSM network like AT&amp;T or T-Mobile. Check this article if you want to read more: CDMA vs. GSM: What's the Difference?.