In BA senior LH pilots normally fly the longer flights, the junior pilots tend to do the East Coast, Central Africa and Middle East. The junior pilots work harder, have more fatigue, but may not do as many flying hours per year.
Comparing flying hours is not everything, but junior BA pilots will normally work considerably harder than Virgin pilots. You will normally be junior at least twice, on joining and when first getting a LH command (more than 2 times if you go DEP, SH command, LH command).
20+ years ago a friend and I left our previous airline at the same time, he chose Virgin and I chose BA. He has earned far more money than me (LH command 13 years earlier), worked less and had far more fun. I chose BA at the time because it seemed more secure. BA outsources every job that they practically can, but that is most unlikely to apply to line pilots (training is another matter). I still think that BA offers greater security but wish that I had joined Virgin all those years ago.