Three are supposed to have the best overall 3g coverage, and it should be getting better too with their mast sharing with t-mobile which is coming in soon.
But really depends on where you are, check the maps, and go for the one with the highest coverage of your area.
http://www.compare3gmobilebroadband.co.uk/3g-coverage-uk.htm
The iPhone isn't renowned for good voice signal (probably not compared to an old nokia, which seem pretty good at hanging onto a signal), although some iPhones supposedly have lots of problems, which might be a warranty issue. It is worth checking if you have two phones on the same network, and one is much better than the other, to phone up vodafone and say, oi, my nokia 6630 can make calls here, but my fancy iPhone won't, and see what they say.
Joe