Depends what you prefer really, I like Nat His, the kids like Science. Science museum is very in your face and modern, it's also has activities on if you have kids with you. V&A over the road though is far better than either of them.
It would be the Natural History for me. You can go just across the street and visit the V&A while you`re there. Quite a lot of their sections are free.
Science museum. I was dragged there by my older brother every saturday of my childhood. he is now a government scientist
I'm not, but I dragged my sons there in the hope of turning one of them into a scientist.