I think you may have a tough time getting an answer to that as it’s a bit general. Lots of differences and lots of similarities. What the “main difference” is I think will vary person to person and school to school. I will say though that some of the
International Schools get crapped on in this particular forum as a few active members have had bad experiences. It’s exactly the opposite in other expat groups I’m in. The local schools take a beating and for the same reason. In both cases if you remove the vocal outliers I believe the consensus is you can get a quality experience for your child at either but that it’s hardly automatic.
My kid is at SAIS (middle not preschool) and our experience has been outstanding. The main complaint I’ve heard from other parents (again removing those with issues specific to their situation) is that the school is very expensive and large. For us it’s cheeper than what we paid back in the states and we feel they do a good job compartmentalizing the grades and classes so that the size isn’t overwhelming.
We have two sets of friends, one couple are PR and the other couple one of the parents is a citizen and the other PR. The first couples kid is killing it and they wouldn’t think of moving her to an
International School. From what I gather they love the both the quality of the education and the cost savings. The second couple has two kids in the system. One of them is doing ok but the other has experienced significant bullying and what they describe and “teacher bias”
Try and check out some of the school forums (or groups of you can find them). I think you will get a much clearer answer in those. You might find your choice may come down to where you can get you child in. Availability may narrow the choices for you considerably.