What a great position - kudos to you for having the 2 offers on the table..
From my viewpoint the big plus for BA is job security, though bear in mind Virgin have a significant presence at LHR. Their slot pairings are worth millions. In the (very) unlikely event they went to the wall, those slots would be fought over by several large airlines, and the aircraft and crews would I imagine end up flying for the winner of the slot war. So IMHO a job at VA is pretty darn safe, just the future of the brand is in question, more so than at BA anyway. Maybe job certainty, as in who you are working for, is a better way to put it.
The biggest plus for Virgin I can see is the 750hrs and rotating seniority. That’s a fantastic draw. We have had guys leave BA LH for VA. Don’t think we get many going the other way? The VA guys I’ve met down route seem very happy with their lot. For the record I’m BA LH p2 and it’s hard work and noticeably so the last year or so. The new rostering system has really thrown up a big question mark over the future and the potential lifestyle we can all expect, especially for the junior. It’s BA’s brexit basically. No one really knows.
Anyway, tough call but a nice problem to have - good luck and enjoy either way!