I can't offer personal experience of the schools yet but we also looked around them both this year, plus Haileybury (secondary), St Nicholas (Old Harlow which is age 4-16) and Howe Green House (primary only).
We had other factors involved as we are planning to move, wanted a countryside location and to secure somewhere for our children through to 18. We are moving our two children to Felsted prep and pre-prep school in September.
Bishops Stortford college was very full and we would have been on a waiting list with no prospect of reliably getting a place for my older one (year 5) except for applying for year 6 or 7 when they add a class. The academic standards are good, they look for top end of average pupils and higher. However, I did wonder how they manage pupils who don't quite make the grade as I got a gut feeling it was stricter on maintaining statistics (of course this is both good and bad!).
Felsted seems a good all round school (and perhaps easier to get into). Extra-curricular appears great - they start to have extended days from year 3 (two afternoons are to 5.30pm) and this increases through the school. I believe they can choose from a wide range of activities on these afternoons. I've met quite a few Felsted families and they have all been lovely too.
My gut feel was they support all the children regardless of ability and therefore there is no stigma or pressure on maintaining grades for the statistics, although of course we all hope for high grades.
I've sometimes read the odd negative comment about teaching or friendships but it's hard to rely on these as you don't know the background of specific situations.
I would consider the entry point and how competitive it is. You can easily speak to the head or admissions office and find out some information to help you understand the reality. Once I did this I felt the choice was easier as I wanted stability and not years of uncertainty and tutoring hanging over us all.
Wishing you the best of luck in your decision!