Matt, do you mean you have had an offer from Virgin or Qantas?
I've only ever worked for BALGW and I love it.
Some friends have worked for Virgin in the past, some loved it, some hated it. Guess someone who works there now can hopefully help you.
Qantas UK has only 3 routes, that's the downer on that one I think. If you're not really bothered about going to the same place all the time, it might be worth considering. At least with Qantas you'll still be in OneWorld for staff travel purposes.
Not much help, sorry, but it's the best I can do. Good luck with whatever you decide to do.