As PP said, the academic 'ranking' is down to how selective these schools are.
Habs is most selective (I have no personal experience though), then St Helens, and lastly Northwood college. Their results reflect this.
We visited and seriously considered both Northwood schools few years back when DD was doing 11plus and thought both were good. The girls we spoke to from both schools are impressive, they both had good facilities, but St Helens is definitely more shiny and had more resources.
We found Northwood College more down to earth and friendly and warm though.
Maybe just the admission process, but St Helens felt more chaotic (lost emails, lost applications etc), very prone to jargon, and a bit too 'large business' like for our taste.
One other thing to consider is that Northwood Girls offers a place in senior school to all junior girls, while St Helens manage some out (hence better result). If you do not want to go through the 11plus process, that maybe a reason to prefer Northwood College.
If we had to choose between these two, we probably would have chosen Northwood because DD felt more comfortable there; we found the school warm and down to earth. Plus we read (at the time) quite a negative review about pushy and competitive St Helen's parents in Good School Guide which spooked us a bit (they are usually not so directly negative about schools)
But ultimately, St Helens is a very good choice too and produce excellent outcome.