I'd use M3 for several reasons. Like you said, the MY is based on the same platform as the M3 so size is close, as well as its power and performance. But probably what is more important is its price.
While the MX is a SUV like the MY (MY = SUV? is another topic for discussion), more importantly the MX much more expensive to buy than the MY, which will probably drive up the cost to cover since they all have the same safety features.
Having said all of that, I have Progressive ins and for me the MX is $0.50 per month more than the M3 if I select the same coverage options. Yes, $3 more over 6 months for same coverage, unless I used different coverage options for each. I chose no loan, own it, so loan payoff, etc. so your scenario and choices may produce a different result based on deductibles, loan, location, driving record, age, etc. I was surprised they were so close. I did not enter in a VIN and there was no option for type of M3/MX like performance or not, so not sure what happens when your actual VIN is entered and it can see what variant you actually have.