My son (age 3) is currently in DMS, which is very similar to IMS. i must say he is learning so much and progressing so well, that it tears me apart to take him out of a Montessori setting this coming summer (he's going to Canadian International).
Between age 2-6, i'd say Montessori is a great environment for kids, especially those who can concentrate well, or those who is able to learn to concentrate (ie. kids with ADHD would have difficulty learning in a Montessori setting, but like my son with mild sensory integration, the Montessori environment actually taught him to focus). If your child is already quite sociable, then she should not have issues in a Montessori setting (the reason i said that is because kids usually do their own work there and less time for socializing; kids who are very shy and quiet may need an environment which encourage them to talk more). The level of Putonghua in DMS/IMS is also known to be quite amazing too. Many expat kids without mandarin exposure elsewhere learn the speak, read and write Chinese within kindergarten age.
If you think your child is intelligent and able to focus fairly, and your end goal is one of the 4 IS you have listed, then i'd go for IMS. I am not sure if Kingston has high school section, but if you are not planning to keep your child there for high school anyhow, then she would definitely learn more in IMS.
Note also that the mass intake for most of the schools you;ve listed is at around K2 (reception), so you may not want to keep your child in Kingston or IMS for too long and miss the mass intake. Trying to get them in at primary age might be even more difficult.
ie. For CDNIS they have 40 spots open for K1 (pre-reception), which are mainly taken by staff kids etc (even kids with sibling priority has difficulty getting a spot). They then have 80 additional spots open for K2 (reception), and only 5 additional spot at K3 (prep). I believe they will continue on to primary without adding spots unless someone withdraws. Therefore, the mass intake is at K2.
Personally i'd love to keep my boy in IMS (he also got a spot there) until primary graduation, as it is a much more solid curriculum than PYP-IB (which is taught in CDNIS), but again, it would be an uphill battle to get back into those top IS if we give up the spot now at K2.
Also, another reason for keeping your child in IMS for now: location. All the schools you listed are on HK Island. Given you don't want to move around too much, i'd suggest that you stay on (or move to) HK Island now so that it'd be easier when she transition into those schools. Best of luck and enjoy her childhood!! 2 years old can be a hand full!!
