The choice of mainsheet is highly personal. I know one guy who's gone through five or six before he found something he liked.
There are numerous trade-offs.
Size - most racers have gone to 5/16" because it runs through the blocks easier. It's hard on the hands in heavy air, though. 3/8" is a little better for that.
Weight - the lightweight lines (Swiftcord, Salsa, Yale Lite) have a tendency to blow / wash off the trampoline in heavy air. On the other hand, they have a great "hand feel". You just have to learn to manage the loose sheet.
I've used Samson Trophy Braid (Dacron, heavy when wet) and it's a pretty good all-around mainsheet. Fantastic hand feel, and it stays put on the tramp. It has a tendency to kink.
I've also used Swiftcord and Salsa (my current mainsheet). I like both of them, although they wear quickly. I learned a long time ago how to manage a loose sheet on the 17, so the problem of it blowing off is not a big deal to me. They run through the blocks without kinking.
I've also used Marstron - it's cheap and works great when it's new, but because it's polypropylene, it wears quickly and gets stiff.
The latest line - Paraloc - is supposed to be
the line for mainsheets. The cover is bonded to the core, making it kink resistant. It's
very expensive, though - anywhere from $2.15 to $2.32 / ft.
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