AT&T vs. Unlocked
I'm looking into buying a couple of Samsung Galaxy s8 phones on ebay and I'm getting the option of AT&T specific phones and ones that are unlocked and are compatible with AT&T, T-Mobile, Metro PCS, anything with a GSM network. Not compatible with Verizon, Sprint or anything using a CDMA carrier. Which is the better option? Is there a difference? I'll need them to work in England on a trip. Also, do I need to contact AT&T to activate the phones? Thanks for any help.