I've had excellent results with both, and frankly use both. I did have a LITTLE trouble the first time I used Polly Scale but I attribute that to just not being used to thinning and using that brand of paint - it took a while to find the "just right" proportions. But once I did, it went on smoothly and looked great. I use MM Acryl a little more often because my closest hobby shop stocks them, but when I'm on the road or ordering on line I'll grab either one.
Coverage DOES seem in my experience to be a little better with Polly Scale, and I'm thinking I'm going to try and start using their flat white for ordnance, landing gear struts and wells, etc because it usually takes a few coats of MM Acryl to get the coverage I need.
My only complaint is, yes, with the shade of ONE color I used - Polly Scale's "USSR Underside Blue" doesn't look like the very pale blue-gray I see in most pictures of Soviet planes. Next time, I'll either mix it to the right shade or buy another color!