I've been in a larger city district (Baton Rouge), a rural-yet-populous school district (in GA), and a small-town school (in LA). Like Fearless said, I guess all have their pros and cons.
It would've been nice to have had a group of school friends I started kindergarten with and finally graduated with, but that was never a possibility in Baton Rouge anyway, due to the aforementioned re-arranging kids throughout the city all the time. I still keep up a little bit with some folks I graduated with, but really, I only spent three years with them, and it's not the same relationship the rest of them have, who all went K-12 together.
Isolating 6th graders seems like a good move, knowing absolutely nothing about the unintended consequences and assuming the money is there to do such a thing. Just hypothetically, in a world where there's no drawbacks I don't know about, it seems to mitigate the weird age that is. Like utee94 said, 6th grade can be rough, particularly for boys, and that was certainly my experience. It wasn't hostile so much as it was just.....how boys are. The testosterone is starting to change you, but it hasn't caught up to where the 7th and 8th graders are, and their hormones demand idiocy, which usually manifests in picking on the 6th graders.
Conversely, I wouldn't want 6th graders in with K-5 either. I remember being in 5th grade and clearly having the sense before school, on the playground, etc., that as I looked around at kindergarteners, 1st graders, etc.....it was getting dumber by the minute that I was in the same spaces as them. Bullying was not nearly so much a thing in elementary school as it was in middle school (in my experience), but it did exist, and some 5th graders were already starting to treat the 3rd graders like punching bags. 6th grade would've only compounded that.
Best to just stick them off by themselves, if you can. Says the guy who has no data, no research, and knows nothing.
High school levels back out again. I recall some amount of upper classmen messing with freshmen, but barely anything in comparison to what goes on in middle school. By then the boys have had a few years with their hormones to figure out to deal with them and still act like human beings, whereas middle school boys.......are just idiots.