If it's boys, it's going to be much higher expectations academically and even sports wise, though Grimsdell/Mill Hill is sporty enough. Is your DS very sensitive or young for his age?
Generally, Habs Boys is a good school for a confident and happy all-rounder at a mid- to higher end of ability - academics, music and sports are all very good. For a sensitive child, competition may be a bit much though I wouldn't say it's intense in early years, they just expect them to put effort and achieve, and you need to want this level of expectations. Going to a super selective from 4+ is not about learning phonics or arithmetic two years ahead of other schools anyway. They mainly build character of high achievers in early years through sports, expectations, encouragement to go the extra mile, resilience, competition, learning to win and lose etc.
Our commute to a super selective school was nearly an hour one way, we used it for spelling and other basic homework. It's fine in the morning but can be tiring after school. Not ideal but manageable. From 7, they go on a coach and just chat with friends.
7+ at Habs used to be much harder as their main intake is at 11+. Not sure if it's going to change much even after VAT as Habs is an academic school reasonably high in the league tables and people are still willing to pay for this.
There are demographic differences too, Habs is predominantly S Asian these days, and families have more conservative attitudes to parenting. This is a generalisation of course, but it's worth considering what peer group you would prefer.
As PP said, you would need to re-assess in any case as children change so much at this age. Your DS may turn out super bright and you might want to apply to Westminster - some boys from Habs go there eventually. Or your DS may want to do football professionally and academic school would not be an option then. It's easier to downshift though than going from a non-selective to a super selective school. And it's easier to apply to Westminster from Habs than Mill Hill.