GFS shouldn't in my eyes be viewed as a back up plan, its our first choice even if our DD passes the Bexley test. A selective cannot offer such a small intake, or she feels the pastoral care of a 100 pupil intake.
There is a huge emphasis on academic stretch/rigor offered at GFS that everyone on this site seems to be searching for. The teaching staff stay with your child throughout their time and will know their abilities inside out by the time the reach GCSE's. Having watched your child grow an develop year on year I'm sure they will want as much as the parents to see them succeed over the finishing line with style.
The two schools you asked about are so very different and both offer great options for your child. I'm loathed to be disparaging of any other school as its about where your child feels comfortable and confidant. My opinions are based on a child currently attending GFS. I am sure that there are equally lots or parents at TT who feel very happy with their child's education.
Happy to talk through life at GFS more if you want to PM me, or alternatively I hope that your receive the new you want next week.