We're in a somewhat different situation, but we recently moved (Florida to Nova Scotia) and a large part of our motivation was thinking about where we wanted our daughter to grow up and go to school. She's doing French immersion here, which was certainly attractive to us. They do like 80-90% French in the early grades, tapering to like 50/50 by grade 8. I speak bad French, and my wife doesn't speak any, so she went in knowing nothing and now (in Grade 1) has learned a lot. She really loves it. A big plus, too, is that I think kindergarten and grade one would have been incredibly boring for her in English. She's a really bright kid, and her English reading and math is already a couple of grade levels above where she is. If she were doing English kindergarten and grade 1, I don't think she'd have learned anything (aside from the social / emotional stuff, which is certainly important).
So, we moved for access to an immersion program (among other things). I do suspect, though, that as others have said -- if your child already is pretty good in Mandarin, they might not learn a lot. My daughter's French pronunciation is excellent, and her fluency (in terms of being able to string sentences together) is really good, but her vocabulary is still pretty limited. I can still understand her books, for the most part (I took 5 years of French in middle and high school). They have a different program here for kids that grow up in French speaking households (although they do put those kids in French immersion if the true French program isn't locally available).