My kids have done both within the same school system (the school moved its 6th grade from at K-6 model to a K-5, 6-12 model) and it has worked fine both ways. From my experience it is a matter of the school environment as a whole. From what I have seen, there are definitely differences in "sophistication" between 6th grade students based on where they go to school, but I haven't seen a correlation with whether 6th grade is elementary school versus middle school so much as how large the school is, how involved the parents are in the school, and especially what the particular group of kids within a grade at said school just happens to be like. YMMV. Too, we have done subject acceleration with my kids in both situations and it has worked either way. Our situation was like yours, though, where the elementary and middle/high school were essentially on the same campus.
I think two things to consider are these:
1) If it is K-6, are the 6th graders protected from exposure to a more "sophisticated" culture? Sometimes yes, sometimes no.
2) If it is K-5, are the 6th graders ready for the increased responsibility and challenge that comes from a different teacher for every class and more autonomy? Some kids are very ready for this while others have a big adjustment that requires more guidance.
