A difficult one, but depends on your age and career aspirations. In BA it will take 20+ years to achieve a 350 long haul command, maybe slightly quicker on the 777. Additionally, at BA you will be at the bottom of a 4000+ seniority list, and joining on the 350 means you will be rooted to the bottom for a minimum of 5 years. It will be at around year 10 that you begin to gain some control of your rosters for destinations and days off. This will obviously be a big consideration if you have children. In contrast I understand Virgin has a rotating bidding system so somewhat better for home life. In addition I hear 330 commands are around year 10.
Regards job security, I wouldn’t bank on either offering you any guarantuees. BA made 200+ redundancies in seniority order with those under 2yrs chopped. I think Virgin did this by fleet, as opposed seniority.
Pay is very similar for year one, but BA gradually overtakes Virgin further up the 34 year pay scales. Just balance this with shorter command times in Virgin for long haul. You can probably get a Heathrow BA short haul command in year 5 after your engagement freeze, but you will work hard and have no bidding control for a very long time.
They are both very different opportunities, but if I was 30+ with a possible young family, Virgin may be a better option. If you want short haul/ long haul, better long term bidding and pay, then BA may work. Either way, it’s a very personal choice, but a very enviable position to be in. I wish you good luck!