On competitive routes, LON (all airports) - GVA, for example the prices are all about the same be it LX or U2, BA etc. The network airlines offer a little more on board, but really not enough to make a difference.
The choices are really 'airport' and then Terminal if you are flying to LGW/LHR. We have found that if we are travelling to the West of London, BRS is often a better choice as you don't spend as much time fiddling around and getting out of the airport. Total journey time can be an hour less!