Scholarship, then Grammar. No scholarship, but if you can afford it, then still Grammar. If you can't, then Ruse.
Grammar offers a wide variety of co-curricular activities (
http://www.sydgram.nsw.edu.au/co-curricular/), and while Ruse offers its fair share, for co-curricular, Grammar is unparalleled.
Ruse, by nature also has a strong maths-science student base, and if your brother is not really into maths-science, but rather history-english-social science, then go for Grammar. Since Ruse is a public school, it WILL have its fair share of crap teachers and facilities.
At Grammar he still WILL have peers with similar academic abilities and interests (Grammar is still fluctuating around TOP 10 in HSC rankings, so if he is a brilliant student, he can still excel.)
Grammar also offers a wider curriculum (
http://www.sydgram.nsw.edu.au/GeneralInformation/academic/curriculum/) than Ruse, especially in the areas of Classical languages and social sciences (e.g Ruse doesn't offer Latin, Classical Greek, Business Studies, Legal Studies) and Ruse also has compulsory agriculture from Yrs 7-10. If your brother does want to socialise with girls, he can do that outside school anyways.
Grammar too has a relatively academic environment, as evidenced by their HSC results.
So don't take this as a Grammar-fanboy argument, but it's just my honest opinion. Good luck with your choices