Mylar is likely to be smoother over the flat areas. Peel ply is likely to be better over curved areas.

The peel ply is more flexible and is likely to be smoother over slightly curved areas. As an example the wing tips on the Singer are eliptical. I suspect that peel ply would work better on the tips.

Concerning the Proud Bird finish, I had a couple imperfections on the bottom of the wing due to the folds in the the vacuum bag. Other than that there were no significant issues. I just sanded out the surface a little.