Optimization: cacheAsBitmap vs PNG

I have gone through the search function, cant find the specific answer I am after.

From a pure optimization point, is it better to cache a movieclip as a bitmap, or to import it as a png? These movieclips are static, no animations or anything. Only there _x value changes throughout the movie.

I hand drew these objects in flash, then optimized them using the shape curves thing. I could reimport them as PNGs if that will help optimize performance - I am having issues of the movie chugging!