Mainsheet cordage
Julie, in my preaching (ranting) I tend to favor Sampson XLS Yacht Braid as the time-honored gold standard for yacht running rigging. And I work at West Marine and am an outspoken critic of New England Ropes. Sampson XLS costs about the same and has a nicer feel and is easier to work with. Sta-Set XS has a nasty habit of kinking in the pile under your feet and in an urgent jibe this kinking tendency is enough to jam on its way out through the cleat and lead to catastrophe (yes it's happened to me). I swear foully at it and remark, 'This rope could kill people. (Yes, I call it 'rope'. Line on boats hates when you call it that.)
But as I work at a West Marine store that is closing, I will take the best deal I can get. Little Diana has just got a new suit of halyards of Yale UULS, an excellent line that used to be carried by Boat US and is now discontinued from WM. So I got it VERY cheap. The mainsheet and main halyard are both Yale Vizzion, an ugly black-and-gold line with .9% stretch at 3200 lbs' load. You don't beat that with anything even close to its usual list price... and I paid like 6 cents a foot for it.
Whatever you use, you should always use the lowest-stretch stuff you can get for main halyard and mainsheet (jib halyard too if you don't have a furler). Anything else can be adjusted frequently under sail.
(I think I bought ALL the remaining Yale UULS and Vizzion in West Marine's remaining stock... so don't ask me if I can get you any on my discount!)
JC2