For now I do not know your answer but I'm also planning to be a pilot in future and I guess we could be friends. Together we could get great answers to our questions.
Definately BA, because in British Airways as a FO (first officer) you get Å90,000 a year and Å120,000 as captain (it depends, you can also get Å150,000 as long haul captain)
The other thing is of course BA is a national airline so its more expensive, whle Virgin Atlantic is a cheaper airline so you will not earn as much (FO is about Å45,000 and Å70,000 as captain) I mean i want to be commercial pilot and work for BA as well so i did a little research. Just to let you know it is really hard to get to BA so good luck.
I think BA pays more. Go to airlinepilotcentral.com for all the pay rates.
I think Virgin Atlantic's better, they seem to have newer planes and hipper service.
BA seems more traditional.