2014 data:
Fortismere: 0.5343 miles
Alexandra Park: 0.5625 miles
So pretty similar catchments actually.
Fortismere: Catchment huuuugely advantaged (1 million £ plus houses), traditionally very competitive to get in - lots of short-term renting etc, meeja boho arty-liberal feel, Outstanding Ofsted, draws from Muswell Hill's top primaries (Tetherdown etc) so the majority of kids enter achieving the higher levels on Sats, of course it gets stellar results at GCSE and A-Level. It's best for bright children with minimal issues.
Alexandra Park: More mixed catchment, drawing from a wider social demographic, so properly "comprehensive" in that sense, but actually getting quite popular - I would say it has the bigger "buzz" as the meeja crew has been colonising the local houses for their pfb's as they get priced out of Fortismere, so the catchment's shrunk dramatically, not as arty as Fortismere, and even though it has less of the intake achieving the higher levels on Sats it does better on the value add (actual teaching), so results are still "good" in that sense.
Obviously there is lots of hype around Fortismere, and if you do want it locate yourself sub half a mile to be sure, but it wouldn't be the end of the world to be offered Alexandra Park instead, both are actually very good, take a tour round the schools and see which ones right for your ds :)