Depends where you live imo.
My son is enjoying Gunnersbury. It is much nearer to us than C Wiseman, which we also visited. He's obviously only 11 so he doesn't miss the girls yet, and is loving the variety of new boys he is meeting, after feeling a bit bored in primary. I like the sports facilities, and there seems to be enough challenges academically, and rigorous marking etc.
If I had lived within 15 mins of CW, I would have considered it as first choice. As it was, it was too far to even be on my list. (45 mins on the E3)
I thought headmaster at CW was absolutely inspirational, and school seemed big bustling, but great. I liked the fact that both boys and girls too.
However, Gunnersbury I chose because it seemed lively vivacious, slightly more academic in tone (may have been misled there, I'm sure CW is for highfliers too) and I liked the layout, area.
So either would probably be a great choice. From our primary, boys at both schools have been very very happy. If you are near CW, and want a mixed sex environment I don't think you can go wrong with it either...