When applying to top colleges, is it more valuable to go to a mediocre high school and finish among the top or a very selective, prestigious high school and finish in the middle to bottom of your class? (both schools having around the same number of students)
Depends. Many top schools are feeders to top colleges. Look at Naviance data to see what you can tease out.
Colleges will look at you within the context of your school and other applicants from your school. For top colleges, middle from a selective hs might be ok if you have something interesting to offer. Bottom will seldom work.
Assuming you preform to your potential in each situation: the context will neutralize the two situations for the most part but the person from the prestigious high school would probably be given the benefit of the doubt. Going to a school that top colleges are expecting to admit students from could help your ethos vs a school where a student gets into an elite college only once in a while. The good high school also provides more opportunities to showcase your abilities and surrounds you with people who can push you to your potential.