I'm only one data point, so you should probably take this with a grain of salt. I went to a Montessori school from kindergarten through third grade, and I can't stress enough how badly that specific Montessori school failed. Teachers were distant, more like babysitters than having any kind of structure. Any project we wanted to work on was fine, but most of us didn't do anything (the equivalent of pretending to work at work). We'd have to keep track of what we did in journals and get them signed by a teacher, so most of us made up stuff and forged the teacher's signature. I don't remember learning any mathematics at all back then. In fact, I'm positive that it wasn't until fourth grade that I got into basic math. I don't know if that's normal, but my father had been teaching me math outside of school from a very early age, so I was still strong in it. It's a very strange school system.

My final observation is that my Montessori school was so small! I was a shy kid, and going to a small school from K-3rd grade, then another small school from 4th-8th grade, did not help me to become a social person one bit. If you do something against the group, everyone will hate you, resulting in more shyness. Even at third grade.

You probably should evaluate your child. Is s/he shy? If so, it may be a benefit to send them to a larger school. I'm not saying that it will cure them of shyness, but it may help.

I'll totally respect any decision you make though :)