Sorry, don't have DDs at either, so most of the below is only anecdotal, but we looked round both and much preferred APS. I know lots of people rave about Fortismere, but we found it complacent and cliquey, almost like it didn't need to try any more. The teachers barely looked at us, existing parents just chatted amongst themselves, and it felt huge and impersonal.
By contrast, APS felt exciting and full of energy, with teachers who genuinely cared about their pupils. We loved it. I have heard that APS is more into school rules etc and discipline, which I also approve of! Have heard that Fortismere has more of a class/racial divide than APS, but that could just be a rumour.
If my DDs had not got into the local grammar school, we would have definitely chosen APS. Fortismere just felt too smug to us, and the other parents in my DDs' years at school also had the same impression. Having said that, others seem to love it, and I'm sure they will be along soon to provide more information. I know DCs at both schools who are very happy.
Would be great if you could see both schools at some point. The best of luck.