Location dependent! I used to be on O2 on my iphone 3G, signal was great in most places, 3G would almost cease to exist in my house in portmsouth but normal signal was still fine. Then switched to 3 when I got my iphone 4, 3G signal was great in my house in portsmouth and so was the normal signal but in my house in southampton the 3G doesn't exist completely, even when I'm out of the house and about 300 meters away the signal would die completely and says no service on my phone.
I'm pretty sure there's a network coverage map somewhere.