I've said it before, but I'm going to say it again (because I've just looked into it again, as I'm due an upgrade and trying to decide whether to jump ship or not): 4G is not the same from everyone. Threes 4G rolled out at speeds not a whole lot faster than previous gen. I'm not sure if it's much faster now, but I'm skeptical. EE's 4G is faster than most people's home ISP speeds. Both have advantages (one's fast, the other's cheap). But simply saying Three is 4G at a fraction of the cost isn't the whole story. (It seems O2 is by far the loser, btw - don't get O2 4G. Something about their network being unsuitable for urban areas, so it ends up being slow and costly - which means Giffgaff 4G will be equally shit).
I'm still tempted by Three. Money is an object in my case. If it wasn't, I'd stick with EE. Having said that, I'd like to see some real data on average speeds before making the decision.