In all honesty I don't think it'll make any difference what so ever, execpt for people who couldn't get a decent O2 signal, those who won't be able to get an Orange signal or people who live in buildings with thick walls or buildings with lots of metal in them.
Calls will still drop when you switch down or up to 3g, just like they did with O2. This is because of the network chagning (O2 3g to O2 GSM & Orange 3g to Orange GSM calls switch seamlessly because there is no network change involved).
3 will have probably just gone for the most cost effective option, after all they aren't doing too well at the moment, aren't doing too well in terms of subscriber numbers and have huge losses.