Personally, I think the differences in results are fairly minimal, especially when you bear in mind that a number of children don't make it into the 6th form at WGSG (I went to WGSB open day this year & was told that 40 boys didn't make the cut this year). Sadly, this is catching on at Parmiter's too. I expect their A level results will improve as a result, but I'm not keen.
However, clearly, we are talking about two good schools where a child who is prepared to put in the work will be appropriately supported & do well. So it comes down to other things; size of site; proximity to home & ease of journey; co-ed or not. Of course different families prefer different things!
Wanner - you are very lucky! If I was you I would put WGSG first, & if your DD fluffs it on exam day, she will still go to a great school. I have a sibling heading for Parmiter's next year - we did the round of open days but Parmiter's was his clear favourite. Personally, I'm relieved - just because I will only have to juggle one school
PS - Kasturi75 - I can assure you that discipline is good at Parmiter's too!