Three's coverage isn't as extensive as oranges, however it's not terrible as three shares T-mobiles 3g masts, and apparently should share oranges too soon.
Three was more reliable a number of years ago when they had oranges 2g signal to fall back on, that deal stopped around the time EE was formed but to their credit I believe 3 has built a lot of masts to make up for that. Still three's coverage leaves much to be desired if you leave city areas.
In summary, three is a good network with very good deals, their coverage is good in populated areas but lags behind when it comes to getting a signal beyond that. As for orange, I live in a town and orange claimed perfect signal, turns out it was terrible and I dropped calls often. My personal recommendation would be vodafone or o2 for signal reliability, but no LTE with this version of iphone