It depends entirely on where you live (and coverage maps aren't reliable either).
There's no real way to tell other than trying the different networks, right?
Will say this, 3 have switched off most of their back up 2G network. If you live in a built up area but unfortunate to be in a strange spot, your STOL. It took months of arguing and letters to get my contract revoked after we moved, I had zero bars service inside the house.
Now i'm with T-Mobile and couldn't be happier, especially since the merger. When I get out of usual limits, it switches over to T-Mo Orange and keep 3G services.