There will be 2 out of 6 classes in P2 MSP will use Putonghua to teach Chinese from next year. 普教中is not applied to all forms. Studenst have to show their abilities in P1 before they apply for the class of 普教中.
What do you plan for your kid's secondary school? I think a good MSP student is welcomed by many first tier band 1 school. I prefer MS as it provides all rounded education to students. In fact, the competition is keen and the school is tough, it is not easy to maintain a good result in MSP. Try to find a chance to talk to the parents of primary students before you make any discussion as they are the real "user".