The difference? short answer: customer service
i was with 3 and all its incarnations for nine years. service got worse and worse until i had to give it up.
one example: they called me while on holiday in Bali about an unpaid bill. I had just paid my bill, so i asked for more details. they promised to call back. Instead 48 hours later they turned off my service. after contacting them by skype, they said i had two bills unpaid, both more than 2 years old !! I said i had not seen any notices, so please send them to me and if true i will pay, but i am on holiday now, so please return my service. It took 45 min. of begging and pleading to get this courtesy arranged. During it, they made me promise to pay for service that hadn't even been billed yet.
They may have coverage, roaming, iphone, blah, blah, blah, but god help you if you ever have a billing problem and need someone with a brain to sort it out for you (like changing from one plan to a lower plan and getting a credit for prepaid fees...). Even if you are a nine year VIP, heavy use customer, you get no service now.
In contrast, i had a problem with smartone, where i transferred a number to someone else, but kept getting billed for it. They arranged a meeting with a live person, face to face, who sorted out the problem and issued a credit to me within a week.
screw signal coverage and iphones, go with a provider that takes care of you when you need it.