If you plan on using the American Club with any frequency the commute from mid-levels is easily a hour on public transportation, and about 35 minutes if you drive. It also isn't a fantastic place for preteens. There is a teens room or something with video games, movies and a snack bar, but I rarely see anyone in there. It is great for younger kids, however, with a lot of programs, a lovely big playroom and a special children's pool.
Regarding Repulse Bay, some people love it, others hate it. It is very isolated and except for a small handful of shops and a couple restaurants and a spa there is nothing there. But, there is easy and frequent transport to Central and other parts of HK. Repulse Bay beach is insanely crowded (and dirty) on weekends and would have to say it is my least favorite on HK island.
Mid levels are crowded, but right in the middle of everything. Not much for preteens to do, however and most of my friends with children over the age of 5 have moved elsewhere, either to the south side or some to Sai Kung/ Clearwater Bay. There are lots of playrooms and that sort of thing for younger kids, but few large outdoor playgrounds.