"Menlo and Castilleja are both great private schools, but you'll pay through the nose in private school tuition to attend either. Casti is all girls, which is a unique experience among this group, so that's obviously something to consider. Menlo has much smaller class sizes than the public schools, which is a particular advantage for students who for whatever reason need more individual attention, and has the extensive resources you'd expect for career guidance and college placement advice, etc. Still, I found everything I needed at Paly, including a large number of the most demanding honors and AP classes in every subject.
Frankly, if you can afford to live in Palo Alto or Menlo Park, both of which are ridiculously expensive, I would choose Palo Alto and send my kids to Paly or Gunn -- particularly if you have more than one. There's virtually no reason to pay private school tuition unless you have a kid with special needs or interests of one kind or another that demand very small class size or an all-female environment. (I suppose another reason would be if you want your kids to mingle with the social elite, becoming members of "society," attending debutante balls, etc. -- that sort of thing does exist in the Bay Area, although on a far smaller scale than in eastern cities.)"