I think that it’s great to have to luxury of debating the merits of BA over VS as options. If you aren’t clear on my meaning, just fly an American airline (any of the them) and it will all come into focus.
Having said that, I have found in my experience that VS is more consistent is striving to make the experience for their customers seamless and enjoyable. BA also offers a good experience for their premium passengers, but always seems to end up playing catch up to VS, surely due to it’s larger size and bureaucracy.
To chose your alliance to VS or BS, I think you have to look at your needs. If you fly JFK-LHR-JFK every week, I’d go with VS. Skipping dinner and sleeping upstairs on VS in your sleep suit is the best way to arrive as refreshed as possible — after a visit to the arrivals lounge, breakfast and a massage.
If you fly a combination of long-haul and short-haul, BA dominates because of its reach and frequent flier program.
Either way, two good choices in a world of great variability.