Hi, I thought you asked this question in another forum already, but I think it's better in its own area - as it's a distinct topic and more people are likely to see it here. So I'm deleting the other comment.
As for your question: I'm not sure I have a clear answer on this one - I feel like writing on wafer paper is never ideal because you can see through it (even when backed, the coloring can be a little uneven). But frosting sheets tend to be a little more absorbent so any ink tends to soak into them and spread a little more. I guess if I wanted really delicate lines, I'd probably go the wafer paper-route since it's less absorbent.
How about you try with what you have and also order some wafer paper to do a side-by-side test?? I know this won't save you money, but it may be the best way to find out what looks best in your particular application.