It depends on how long you want to keep the sail. The Dacron will go to tatters in 4-8 years, depending on climate and sailing season (some of these folks are from the north and store the sail inside in the winter--I don't and you don't, so the life will be far less) and it will cost $500-1000 to replace. Sunbrella will outlast the sail.
Unless you're a racer, if there is so little wind that it makes a difference--we're talking 2 knots--you've turned on the engine anyway. The difference is small, IMHO (my current genoa had a light cover--I noticed very little difference with Sunbrella replacement, other than I'm not worried about it anymore).
If you replace a sail when the shape goes just a little, it won't matter. I can't imagine deciding based upon looks.