It's a close one to call in our view, the wafer paper vs frosting sheet debate! As Julia alluded to here and in her video, it really depends on the purpose.
Here's our thoughts:
If you are wanting an image to be a centrepiece of the cookie I would opt for icing sheets, since the image is generally crisper and usually a little closer to the 'true' colour.
Otherwise, if it for decorative purposes (eg leaves, butterflies and some patterns etc) then wafer paper is suitable, since its rigid nature helps to hold shape better.
We've done a summary post on this, which reviews the different characteristics based on material, printed appearance, application, taste and price. I hope it helps you all: