Pro VS:
- You're a 'First Class' customer (since there is no First, Upper Class is as good as it gets)
- You're quicker off the plane, being in Front of it.
- The Lounge at LHR is much nicer than BA's Galleries (Club or First)
- VS is simply 'cooler' and more modern, Bar on Board, top Service etc.
Con VS:
They don't fly to LCA (or do they? All VS flights I've seen were Codeshares operated by Cyprus Airways)
Pro BA:
- The Alliance / Network (with Oneworld, AA,CX,QF,AB) offers considerably more destinations and hence easier ability to achieve and use Status, as well as to earn and redeem miles (assuming you are okay with the fact that there is no such thing as a "free" flight.