For VS
- In my experience BA crews are extremely hit and miss. I've had some excellent crews but I've also had a slight majority of crews who were at best indifferent and at worst outright rude - these days I avoid BA wherever possible for that reason. I don't want to pay a lot of money to be treated like I'm a nuisance. VS is much more consistent in terms of good crews in my experience. And that is only counting business class crews.
- VS has the much better lounge than BA at LHR - the T3 Clubhouse rivals most First Class lounges let alone Business Class.
- You're guaranteed WiFi on VS whereas it's still more of a mixed bag on BA as to whether your plane has it and whether they activate it.
For BA
- If you can get a window seat with direct aisle access the seat is probably better than the VS one for most purposes.
- Lots more schedule flexibility.
- Upper Deck on the 747 is a particularly nice experience.
- If you get a good crew BA can really be one of the best experiences in the sky (but as above in my experience the odds of this are not great).
Overall I'd go with whichever was cheaper, and if they're the same price I'd personally fly with VS for the better overall experience (including pre-boarding.)