I, too, have strayed to the dark side (well, red side, actually). It is a close run thing to answer the 'which is better?' question:
I love the VS relaxed atmosphere - sitting at the bar in 'ninja suits', the 'finest British steal' [sic] on the butter knives and the 'pinched from VS' on the salt and pepper pots. Clubhouse food is better than Terrace offerings (for CW / silver pax) and the limo service is superb!
However:
BAEC is (IMHO) better than Flying Club - benefits and miles
BA still provides that feeling of superiority (There'll be some comments about this!) - I am a traditionalist, and I enjoy that 'little piece of UK' feeling as the strains of the Flower Duet strike up.
I also feel that BA has more 'clout' in a tight spot - I have no evidence of this, just a feeling...
I am looking forward to my next BA trip which will be my first out of T5 - I shall, perhaps, know then, which I prefer...