They're much of a muchness. Choose depending on where you want your frequent flyer miles to go.
There are loads of threads on this. My 2 cents is that BA has better service and a better overall attitude to its customers. On some VS 747s, PE is in the bubble which is nice (BA's is always downstairs behind Club) if you can get your seat there. But I racked up a lot of miles in both: for me the seats, IFE, food, lounge access (or lack of) etc etc are pretty much the same: I go for BA for the FF miles, the better route network and the service.