It's all a matter of personal preference. You have to weigh the pros and cons. Everyone is giving great advice here.
Here's my advice: find someone that has an M1 that is similar to your ideal setup (turbo, susp., brakes, ect.) Ask them if you can test drive it. Then test drive the MS MX-5 (when they are available). Then make your decision based on how each car makes you feel.
Reason for my theory: Some people say the M1 (once modded) feels the tightest in the turns and quicker when turbo'ed.
Then other say that the M2 feels more set and stable like a true sports car and that the suspension changes/added weight (as compared to the M1) are well founded and worth it.
it all depends on your preference. It's a big step to buy a new car or dedicate your current one to modification. Just see how you feel
in each one so you will
not regret having made a decision without experiencing each first.
Good luck in your quest for mo' powa!