For late 80's and early 90's posters, when there were fewer DS posters printed as a proportion of the total number, I think there is a price differential which will of course vary depending on the popularity of a title and scarcity - Goodfellas and Shawshank Redemption are two titles I can immediately of where there is a big premium difference. If a DS unfolded poster is available for a title I want, that is the poster I will look to buy. I prefer unfolded SS posters over folded DS posters.
Mark