What is the difference between acetate sheets and baking paper when making chocolate deco?
Acetate sheets posses a firm and rigid shape, with a glossy coating that assists the removal of decorations such as chocolate decoration. It also can be shaped without being indented at any point which baking parchment most frequently fails to do.

Baking parchment is very agile, this fails to support decorations as a mould, and would consequently lead to breakage of the decoration should the parchment not retain a flat edge. It could also bend in frequent points, misshaping the chocolate. This is what makes it suited towards cakes, as as it can be ripped off.

I would therefore conclude that it would be most suitable for you to use acetate sheets.