It really depends on the look you prefer. You can pair either the large or small plain circles with the scalloped layer. It totally depends on whether you like more or less of a margin around the plain circle.
The photos in this post give a good visual representation of what I am trying to say:
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So, for a small margin get both sets small or both sets large. For a larger margin get the large scallops and the smaller circles.
Personally I got both sizes in the plain circles and the larger size only in scallops, which gives me the option of both. I really haven't missed not having the other set of scallops at all.