Personally I absolutely hate the taste of fondant sheets & so does everyone else I know.
Personal tastes aside I do use fondant sheets on request, I would only recommend them if you are already doing a fondant cake & only for big images. But I would actually use Wafer Card for the large images (or wafer paper works well too)
For me it is wafer paper all the way & Vanilla over unflavoured all the time, I even use vanilla on savoury items (but then I am a little obsessed with vanilla 
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We are able to get images nice and clear on wafer paper. For larger wafer designs when I want them to stand up I will use wafer card. These work best on the side of large cakes when you want the image to stick up over the top of the cake slightly.
Okay, but you may be a little biased, I think.
On a more serious note, a question: what is a "fondant sheet"? We use the terms "frosting sheet" or "icing sheet" here in the US, and both are typically made with tapioca starch and sugar; they are not thin layers of fondant. Also, what are "wafer cards"? We typically only see wafer paper here, which is pretty sheer and brittle and allows any color behind it to show through.