Firstly, congratulations to your son. Your question is slightly difficult to answer. As you might now, Habs' test papers are quite inconsistent in the levels of hardness, though are generally set to challenge. In maths, when I sat the paper (that is in 2010), I would say that Habs was harder. At St Pauls, if you knew the most advanced parts of the syllabus (id est - algebra), your son would perform well.
Their test paper questions are of very high standard and are based on the book, So you really want to learn Maths Book 1. However, the test I found difficult - out of all the papers (that is, John Lyon, Watford Grammar, City of London, St Pauls, MTS and Habs) was the, undoubtedly, the HABs Maths paper. HABs maths had everything in the curriculum but you had to be first- for they would give very challenging questions. For example, in my year, they taught the 'base' number system - which you had to apply your knowledge to, and can be quite time-consuming. I found the penultimate question so difficult to think through, I left it out!
The Habs English in my year was of 13+ standard and their marking scheme were very pedantic (and according to our English teacher, 1 mark was lost for each grammar, punctuation, syntax error!) However, last year's examination paper seems to be fairly easy (though that compensated into the Interview, which had tougher questions). St Pauls' English was by far more difficult and that explained why almost 1/2 the pupils did not get into the interview round.
It depends, really. At Habs they did examinations to find who had some talent (and when I first started Habs I was quite surprised at the slightly low-standard of my friends in the class - but because of their skill they're all incredibly clever now). At St Pauls, their examinations was more corely to find the creme de la creme (apologies - their isn't a tool for French accents!).
St Pauls is a very good school, without doubt. Their interview questions, to me, started with some English (where I discussed reading through The White Tiger and we talked a bit about the Man Booker Prize). It was done by a friendly teacher, so your son will probably be encouraged by that! However, the Maths was done by a slightly intimidating teacher.
He asked the following mental maths questions -
24 x 25
A gelosia-strip grid
Applying your knowledge to a Venn Diagram
And the square root of 0.09
Since I panicked on the first one, which was really quite simple, I did slightly badly on the other ones - but my love of the school, English and the former test results helped me to get an admission there. There interview questions were not done in a group.
For HABs (if you are considering), they had both a group and single interview, and meanwhile the teachers talked with the parents (which probably also took count). In the group interview, we were given this picture: http://www.luminarium.org/renlit/henry8pope.jpg
We were told to analyse it. We were then asked about who our hero was.
Then in our single interviews, we read a passage and were tested on things such as vocabulary, general knowledge and cheerfulness (the most sobre of people from my class or the most dull did not get past this level).
Hope that helped and hope to hear from you soon,