Sailing on the river I would go for Dacron. It lasts longer and I think is easier to read. It lasts longer as the laminates on a Mylar sail will shrink at different rates. I think this outweighs having a lighter laminate sail. For the river I would go for a flat cut. [Interestingly a laminate, if you go that way, when shrinking will likely become flatter too. It will hold its design shape very well initially but will then turn into a bit of a car crash...]