I found their customer services very helpful. Yes the call centre is based in India but I've never struggled to understand them and vice versa. I choose 3 on launch day for my iPhone 4 and I'm very glad I did, personal hotspot and USB tethering included.
I'm going the other way, Three was great for me in town and when i was in Uni accommodation - but I get zero signal at my parents place.
I don't use many minutes so I'm ditching my £20/month contract for a £10 a month GiffGaff goodybag. Contract is up this September but I'm going to phone up and see about early cancellation.
Excuse the ignorance but aren't they the same thing effectively?