The other exhibition I saw - last week - that was really interesting was this one on Zaha Hadid, the architect at the Design Museum near the Tower of London (but opposite side of Tower Bridge)
http://www.designmuseum.org/exhibitions/2007/z ahahadid
The most amazing and unusual building & furniture. There is also a Matthew Williamson show but I'm not a great fan of his. The dresses are so flimsy it's amazing they lasted the evening! They certainly won't last long enough to be shown at the V & A in 50 years time!
Depending on your interests I think the V & A is best.