The one situation where it does make a difference which company you use is if you are going to very remote places. I use Orange and I know of one place where I can't use it (on the Sound of Iona in Mull) as there is no Orange transmitter that isn't blocked by a hill - T-Mobile does work, but in other places the situation can be other way round. If you are going somewhere far away from cities with rough terrain it might be worth checking with a local as to which service they use.