If you are sailing in a primarily windy area, I'd go with the 5oz dacron main. Better durability and less stretch. The only time lighter dacron would be better would be if you were in an area with light winds.
We had an 1988 H17. It had the Neil Pryde Mylar sails like the Hobie 16's of the same vintage. A few big issues:
1. They were cut very shallow. Light wind was terrible.
2. Delamination. After three years of use, the bottom 4 feet of the sail was in horrible shape.
3. There would be spots where the sails would never dry.
4. They were pin-head designs. Without the flat top it was tough to spill air without traveling way out.
On a side note, Pentex is only 10-20% higher priced now-a-days compared to Dacron from a materials standpoint. Places like Calvert that are charging $1300 for a flat-top Pentex sail are way over priced. Shop around.