I stand to be corrected but I think Silver on BA takes a lot more flying to attain than the 15 tier points on VS.
The reason I voted for the Flying Club, and the reason I have nailed my colours to the VS mast are :
Before I attained Gold, the big attraction was the fact that TPs and Flying Club points are awarded in every transatlantic class of ticket - this isn't true for all airlines - especially the TPs.
The other good thing was the availability of reward flights. Twice I was able to book 3 x UC reward seats to the west coast of the USA during school holidays. From what I have read elsewhere, this is very unusual.
I appreciate that UC reward seats on the LGW routes are an issue, because there are so few UC seats on each plane, but that has never bothered me.
Once I had attained gold, that is where the benefits really kicked in for me. I do not fly UC, unless they are reward seats, so the CH access and the very high percentage of op-ups I have been given means the AU card is worth its weight in gold. [:D][:I]
The Flying Club isn't perfect (the Amex card proves that), but reading the other forums on Flyertalk,and the moans that other airlines frequent fliers have, I think it is the best of the bunch - for my flying patterns.
YVMV [;)]
Nick