The New iPad from USA

[hw007]

Hi,

I'm from the UK and I am planning a trip to USA in September. I have decided that I am going to buy the new iPad 64Gb Wifi+4G.

Which one should I buy?

AT&T or Verizon?

Mind you, I don't plan to use a cellular connection here in the UK as yet, but I might want to. And I also know that the 4G frequency is different from the US.

1. So I need to know which one is the best one for the UK, AT&T or Verizon.
2. Will it be SIM Unlocked (when and if I want to use cellular)

Thanks

HW