Well this thread took a turn!
The advice I'd give to any family looking at around at reception age is to know that this choice is important and that once you have chosen, supporting your child's school is your next important job because in the 7+ years that they spend in that school you can expect all kinds of curve balls in your personal and professional lives. Some days life will just chew you up and spit you out. The staff in these schools will hold it together for you when times are hard and your children will learn, play and be cared for on days when you can barely order a deliveroo. Teachers are your ultimate wingladies and wingmen. Moving children from school to school can be important when the reason is centred upon the child's needs. Please do visit the schools at least 2-3 times before making a decision.
In answer to the original question, I have children at Greenfield and friends with children at HB and by all accounts, we have very happy and thriving children. The best advice that I received was to look for a school that would celebrate my children for who they are. There are differences in the cultures at the schools that could make one or other a better fit for your family. Both schools will continue to be successful because they help to offload the pressure on families with two working parents by providing after school clubs and holiday clubs.
I won't say a lot about what is special about HB because I don't have personal experience. From what I understand of HB compared to GF, they lean more towards competitive sports, their testing strategies are different and the hours are too (notably from Year 3). The age ranges that the schools accept differ and HB are expanding to a senior school as well.
Greenfield are unapologetically diverse and inclusive. Form sizes are 24, core subjects are taught in groups of 12. Our children have welcomed so many new friends during the expansion of the school and are kinder humans for it (kinder than they would have been with the experience not kinder than any other child of course!) They had zoom time with their teachers and friends every day in lockdown. The performing arts department is something to behold if you want your children's confidence to blossom and you might even find that they are presented with opportunities to perform at venues such as G Live. The SEN department is an asset and a toolkit for children that illustrates the complex understanding GF have of childrens learning needs. In my experience resources there are available in situations led by the child's needs; such as during challenging life events, if a child is particularly nervous before a music exam and to maintain the stretch in the children performing at the top of the academic curve as well as the bottom. Finally, the headteacher might be the single hardest working human that I have ever met. If you do have the privilege of meeting her, please ask her the question of why it is important to her to create the school that she is.