Hard question, Frankie..
I personally, won't ever use the 3 Network - been there, and never again will I spend money on them. Still get pestered by them now, to take out a new contract.
I used to be with Vodafone, they are a good overall network and feature rich supplier - but expensive on calls. However, I switched to T-Mobile back last year - haven't had a single problem yet, apart from their 3G coverage in my area, which seems to be better now. My wife is with Orange, again never had any problems with them either, and they offer free broadband depending on the tarriff you chose.
I'm personally, staying with T-Mobile when I upgrade my phone in July this year, I pay just under £40 a month but that is mainly for a web-based package, 40Mb download/upload transfer, 200 Free minutes and can't remember how many texts a month. With the tariff I'm on I can watch TV, with no problems if I'm in a 3G coverage area.
So, its not an easy question to answer - apart from avoid the 3 Network at all costs!!!
