My warning to be to make sure that the other students are similarly advanced. A cluster only works as a cluster if it's really a cluster. It might help in teacher selection (ie, getting a mathy teacher who's good at differentiation) to put all "advanced" students together, but if the students are working 1,2 and 3 grade levels ahead then they'll most likely all be given work that's one grade ahead.
I was going to say the same. Why were you the only family given the option of subject accelerating to the middle school? That leaves me wondering if your ds is actually more advanced than the others who would be in his cluster were he to stay at the elementary.
Beyond that, looking at the difference in the curriculum btwn the two options and how good the teachers for each option are, as a pp mentioned, would be what would sway me one way or another.
