Aaaah insomnia!!!
You are not hoping to get an objective answer surely? MHS alumni will tell you 'Go Mang'u', Strath alumni will say 'Go Strathmore'. I doubt anyone has studied in both to give an enlightened answer.
My biased piece: Mang'u is a school of hard knocks located in a semi arid area with a strong focus on pure sciences and maths. Rugby, basketball and hockey do well (or did well, its been a while). The main pull was or is Aviation Tech; a kid who wants to go into arts, law or finance might not find many peers. My year, only 2 did law, about 10 BComm, another 10 BA,...the rest out of 120 did BSc, Engineering, Medicine, Pharmacy, Architecture, Comp Sci and the other assorted annoying technical courses where you get employed by rich entrepreneural BA people who you initially thought were dumb

My politically correct piece:
Request a short tour for the kid with the administration of both schools, whichever the kid fancies, enroll there.

It also boils down to whether your nephew wants to be a boarder or a day scholar.

I wouldn't turn down a chance to go to Strathmore. Mangu has been steadily declining performance-wise over the last couple of years. Strathmore looks like it has been improving. Look at 2013 results for example:

Pos. 7 20406020 STRATHMORE SCHOOL Private 78 A- 77.060 NAIROBI

Pos. 26 11200003 MANGU HIGH SCHOOL National 211 B+ 72.792 KIAMBU

Of course there are other things to consider; Day versus boarding; Cost etc. I would choose Mangu if the national rankings didn't particularly mean much to me.

I vote for Strathmore any day.

There you get quality and discipline, consistently.