I posted a variation of this last year in one of the more general forums (can't remember which), and felt like many of the responses I got were from people who hadn't actually experience parenting older children yet, so I'm going to try again here.
My son attends 5th grade at a small crunchy charter school in an urban area. He could conceivably stay there for middle school, but for a variety of reasons I'm not sure I want him to. Right now the prime contender to replace it is a large public middle school in an affluent inner ring suburb -- we'd move to be in district.
One fact that plays in my decision (far from the only one, there are huge differences in demographics, curriculum, and extracurriculars) is the question of what happens afterschool. Here are the options the way I see it.
School 1 (current school): He would stay in school sponsored afterschool activities each day until I could pick him up around 6:00. The activities aren't things he'd necessarily choose because the options are limited. There have been several murders of children afterschool along the route he'd have to walk home. Walking home will never be an option in my opinion (or at least not until he's in high school which would be a different school anyway). The way I see it the pros are that he's supervised and busy/out of trouble. The cons are that he'd be arriving home around 6:20, after a 10 hour day with homework he needs to start.
School 2: Afterschool options would include riding the school bus home to an empty apartment (the dog would be there for company) arriving at around 2:45 and staying alone until maybe 5:30 or 6:00, staying for afterschool "clubs" for an hour and a half, and then riding the school bus home and staying alone for an hour (this is only an option T, W, Th), walking or riding the public bus (in a very safe neighborhood, about a mile and a half) to Grandmas (not every day, she's pretty busy), staying afterschool to 6:30 with kayaking or rock climbing club (once or twice a week) and getting picked up, or walking a mile (again, safe walk) to the subway and traveling to his best friend's house or Tae Kwon Do classes, and having me pick him up there. Pros: He'd love all these things, the clubs and activities are things he enjoys, on the days he came home he'd have a shorter day and could start on homework or recharge his batteries. Cons: How do I feel about my 11 year old walking around the city, riding the public bus, and being home alone after school? In later years he would also have the option of staying and practicing on a sports team, although I'm not sure he'd pick it. Sports teams aren't open to 6th graders.
He likes the idea of coming home on the school bus to an empty house, and he likes the array of afterschool classes at school #2, but he isn't too sure about the public bus, the subway, or the long walk. I'm a big believer in listening when your kid tells you what they're ready for, so that's a factor.
He's a very sweet, super responsible, well behaved kid, with a lot of "wisdom" and common sense.