It's a very difficult comparison to make.
Which class? Which aircraft type? Where am I trying to get to? Leisure or business? Casual or regular?
VS has an extremely limited route network, really only making it useful if I'm headed from London to North America.
For most aircraft types, if I'm flying in economy, there's little difference. They're uncomfortable in both cases. I marginally prefer Heathrow T5 to T3. BA's 788s are a disaster, so I'll avoid like the plague (I'm assuming we're comparing long-haul, as short-haul makes no sense in the comparison).
VS has a better hard product in premium economy. Their business product is great if you're travelling alone, but poor if you're with somebody. BA's business product is solid, but their policy of charging non-status customer to select a seat is ridiculous.
Soft product is variable in both cases. VS put a gloss on, but can be very unprofessional when things go wrong, and the whole Virgin "yay, cool, let's do it" grates with me. BA have lost their professionalism over recent years, which is what happens when the industry in the UK do their best to turn cabin crew from a respected profession into a minimum wage job.
So, I'd generally choose neither, given free rein. But for my flight patterns, Virgin generally just don't cover the routes, so it'd end up being BA.
