At the time I was selecting a preschool, it wasn't a specific study, however interpretations of various studies.
If it's helpful, this is a more recent one: https://srcd.onlinelibrary.wiley.com/doi/10.1111/cdev.13575 which shows that Montessori preschoolers performed comparably to non-Montessori preschoolers in Kindergarten, though "disadvantaged kids" who attended Montessori schools performed better than "disadvantaged kids" who did not. (My interpretation being the correlation to performance is that the parents got their kids into a Montessori preschool despite economic disadvantages--biased because of what I learned in Freakanomics).
And a tangential analysis: https://www.nature.com/articles/s41539-017-0012-7 that concludes that most studies around Montessori are not sufficient to draw any meaningful conclusions, however adapting Montessori methods into traditional classrooms appears to be beneficial.
Regarding your upcoming decision--if I can offer my advice: find a preschool that best meets your families needs, and not worry too much about academics (at their age). If your children enters grade school with the abilities to succeed in the classroom that's great. If not, it's a great opportunity for them to learn early age how to overcome challenges--which will serve them well when they are adults.
If it's helpful, this is a more recent one: https://srcd.onlinelibrary.wiley.com/doi/10.1111/cdev.13575 which shows that Montessori preschoolers performed comparably to non-Montessori preschoolers in Kindergarten, though "disadvantaged kids" who attended Montessori schools performed better than "disadvantaged kids" who did not. (My interpretation being the correlation to performance is that the parents got their kids into a Montessori preschool despite economic disadvantages--biased because of what I learned in Freakanomics).
And a tangential analysis: https://www.nature.com/articles/s41539-017-0012-7 that concludes that most studies around Montessori are not sufficient to draw any meaningful conclusions, however adapting Montessori methods into traditional classrooms appears to be beneficial.
Regarding your upcoming decision--if I can offer my advice: find a preschool that best meets your families needs, and not worry too much about academics (at their age). If your children enters grade school with the abilities to succeed in the classroom that's great. If not, it's a great opportunity for them to learn early age how to overcome challenges--which will serve them well when they are adults.