Well, let's compare 3/16 Sta Set (which is not a pleasant line to use) to 3/16 Samson Ultra Light (simply because the specs were easy to find). Both are about the same diameter. Both have about the same tensile strength (1400 lbs vs. 1200 lbs). Sta Set is double braid polyester. Ultra Light has a HMPE core with a polypropolene cover.

Just to make the comparison easy, say you pull on your downhaul line with about 130 lbs force - that's probably about twice as hard as I ever pull mine. At 10% load, Sta Set has 1% elongation. Ultra Light has 0.5% elongation. If that difference (one half of one percent) additional stretch is noticeable and bothers you, then by all means, go for the high modulus line. I myself can't tell the difference by feel. I simply pull until the sail looks good.

Of course, I'd still choose Ultra Light over Sta Set -- because it is easier to grip. For a continuous loop, I'd choose something easy to grip and easy to splice, such as Marlowe Excel Control (which is specifically designed for that purpose).