There are two types of elongation: elastic and inelastic. Elastic elongation occurs when you tension and then de-tension a line. The amount that the line springs back when tension is released is the elastic elongation, which is also called "stretch". Inelastic elongation occurs when a line is first tensioned (when new, or after a period of disuse). The amount that the line lengthens but does not return when tension is released is the inelastic elongation, which is also called "creep".

All cordage has some degree of both stretch and creep. The amount of each varies with the composition of the fibers and the construction of the rope. Nylon, for example, is quite elastic (which makes it good for anchor line, but bad for control lines). Polyester (Dacron) is relatively inelastic. HMPE (such as Dyneema/Spectra) is much less elastic than polyester. HMPE, however, is very slick. Even in braided lines, the strands will slip against each other under load (tightening the weave). It has a lot of creep - much more than polyester.

Have you ever noticed that your shrouds (even 1x19 wire rope) feel tight at the start of the season? Or that they seem longer after a heavy-air day? If so, you've experienced creep. On the last jib I made for myself, I decided to try substituting 1/8" Amsteel Blue (Dyneema) line for the wire in the integral forestay. It is light and plenty strong, but it crept like crazy. The stay is only about 20 feet long and after the first use, I cut it down and respliced it 6 inches shorter. By the end of the season, it had elongated another 4. I'm building another jib now and will go back to a wire forestay.

Marlowe Excel Dinghy Control line has a blended fiber cover over a polypropolene core. Given that the cover will elongate by itself, and will lengthen more as the core compresses, I'm not surprised that that it has lots of initial creep. That's the price you pay for line that's easy to put in a constant diameter fully covered end-to-end splice. Tension it well (with a block and tackle between trees) to pull out the creep before splicing.

My mainsheet is an example of the opposite end of the spectrum. It's a piece of 5/16" Samson Trophy Braid. Now, Trophy Braid is one of the stretchiest double-braid polyester lines available. At sheeting loads, it might experience an inch of stretch but I can't feel it as I play the sheet. I use Trophy Braid because it has a soft, fuzzy, easy-to-grip cover that holds in the cleat very well. The main downside is that it retains water (which adds weight). I'm occasionally tempted to replace it with Robline Racing Sheet Pro (which has an easy to strip cover) but I honestly don't think that would make my boat any faster. In fact, I did make myself a set of Racing Sheet Pro jib sheets, sailed with them once, and went back to my polyester jib sheets.

In the end, most any modern cordage is strong enough for dinghy/small cat running rigging, so the final choice of line comes down to other factors including spliceability, one's tolerance for creep and stretch, cleating security, grip, and ease on the hands. And yes, many of these are matters of personal preference. I agree that it's nice to look at and feel the different brands of line (if possible) before buying.

Regards,
Eric