Both my children go/went to APS, eldest with an EHCP and APS was perfect for him. We did visit Heartlands but the SEN provision then (7 yr ago) wasn't right for him. Youngest has just done GCSEs and hopes to stay on for sixth form.
I think it is an excellent school, great communication, excellent pastoral support, brilliant teaching. The only thing I didn't like was the strict uniform policy - but I know I'm in a minority of not liking school uniforms. My youngest didn't like a couple of the teachers but I'm pretty sure that's the same in all schools. My son was bullied a little (by another SEN child) but this was dealt with quickly and well. They are strict and my youngest sometimes complains about 'the rules' but I think this strictness does help with behaviour in a school full of teenagers.
The 'catchment' is tiny though, less than 0.4 of a mile. I think that distance went up slightly once past the acceptance deadline and some people didn't accept the place, but I'm not sure if that's still true. We got in with an EHCP but even 7 years ago we had to appeal. At the time it was the only school that could provide the SEN support my son needed, and I looked in surrounding boroughs too. I'm glad to hear Heartlands SEN has improved, I really liked it when we went to look at it.
I know a couple of people with children at Highgate Woods and they are happy there and doing well. We went to look at their sixth form and I agree with the earlier comment of the look of the science block, but I was impressed with the science teachers we spoke to.
I know Ofsted has massive problems atm, but both Fortismere and APS have been re-ofted'ed recently which is probably a good thing after 14ish years since their last one.
I'd recommend going to see as many as you can fit in, and go to the evening and open days. It's the best way to make up your own mind about a school, the open days give you a more realistic view of the school during a normal day. You don't have to wait until year 6 to start looking. I don't know the exact timings but the council look at how long you've been at an address to discourage people from temporarily moving to get a place at a school.