Are you suggesting that Heath Mount is a better school?
Like I said, it all depends on what you have in mind for secondary and how academic your child is. Heath Mount is more rigorous and also prepares children for boarding schools as well as day. They do send a lot of children to the local independent schools eg Bishops Stortford College, Haileybury & St Edmunds but not exclusively. BS and St Edmunds also have their own prep schools which I notice you haven't mentioned, but perhaps that is because of distance. If you are considering local independent schools is there any reason why you're not considering these schools? perhaps you don't want a school that goes through from age 4-18yrs? or perhaps you just don't know about them?
It's not as easy as saying one school is better than another, it all depends on what YOU are looking for.
Without knowing anything about your preferences and your child, St Jospeh's in the park and Duncombe are 'nice' schools. They are not the most academic in the area but will probably get you into a local day independent school but they do send a lot of children to partially selective comprehensives in the area eg Simon Balle, John Hale, Broxbourne school. I wouldn't send a very academic/heading for grammar school dc there. Broxbourne like the previous poster said, is not a grammar school area, so they are not geared up for this.
I would advise taking a look at their 'Leavers destination' page for each prep to see where their student go to when they leave to steer you in the right direction. However, from experience of this whole process, the schools who aren't terribly academic tends to bunch all the schools they've EVER sent children to under one heading, 'Common school we send children to", which is euphemism for we don't send children to the top schools regularly but we have sent some before. The more rigorous schools tend to lay out actual numbers of children they have sent to which senior school for each year.