If your daughter is happy at Weald I would leave her there. My eldest left Weald for university last Summer and I have a Year 9 at TGS. (I've also got a Year 12 at Hillview for good measure). I therefore have a pretty current overview of both schools and think they are both very good.
My daughter moved from the Sevenoaks annexe to Tonbridge for 6th form. She got excellent GCSEs and A levels and is now at a very well regarded university. I am pretty certain she would have achieved the same whether at TGS or Weald. What I do feel is that she grew in confidence at Weald.
Remember that the reason TGS is higher in the league tables is because it takes the highest scorers in the Kent test. I would look at the value added by the school as well.
My year 9 loves TGS and it is a perfect fit for her. I actually think that, certainly up to now (it might be different in year 10) the girls receive far less homework compared to Weald. TGS so far has been nurturing, not pushy.
As others have said, at TGS the girls study Latin, Spanish and French from year 7, so you will need to consider that.
There is a lot of movement between all schools at 6th form, so this is another option.