yeah ive met a few people whove gone from private schools to select entrys (and a couple who went back!) and i reckon i would pick haileybury over macrob (ignoring money (which is a huge factor but im just gonna ignore it)).
unless it was for year 9 entry at macrob and there was no scholarship at haileybury in which case I would go to macrob(i couldnt ignore money for 1 line LOL).
just a quick aside - the main point of the select entry schools is mostly for social mobility's purpose. if you cant afford a good private school but you are a good student then their existence allows the bright kids to get in and do their schoolwork in a great environment with more resources than most/all regular public schools. if you are coming from a private school that is elite and you either have a scholarship/can afford easily, then there isnt a whole bunch to gain from leaving it to go to a select entry school. opportunities across both schools are probably similiar, but overall the private schools probably edge out the select entry ones simply because they have so much more money and resources. this is all especially true for APS schools like haileybury. if you already go there and you are happy then just stay.
TL;DR: academically macrob has more smart people, but at the top end of the spectrum haileybury and macrob are probably quite similar (the smart kids at haileybury are really smart and often get scholarships),
for extra curriculars; haileybury is probably the winner. the aps schools just have so many resources for extra curriculars, especially in areas like the arts, and outdoor exploration and shit. arguably sport too because the good private schools have their own sporting league and shit (although mhs is probs on the same level as them).
for diversity - mac rob definitely has more non-white people, but that doesnt necessarily = diversity. i know in my cohort at select entry, there were like 5 main nationalities (viet, chinese, indian, sri lankan, pakistani) that easily made up over 70% of the population with those leftover coming from a pretty wide range of backgrounds. i think this is pretty standard across most select entry schools
either way you cant really go wrong, good luck!