Racing in a scow, there's probably lots of "psych-out" stuff that people have: super thin kevlar this, and super cool purple spectra that. For some of the controls, like the outhaul, for example, it may be important to have as little stretch as possible in the
line. For others, (centerboard pennants- I don't even know if MC scows have them) the
line will be so short that it's not important-- anything you find will work. One caveat with the new "super" synthetic
lines is that it's easy to go to thinner stuff, trying to save weight. The problem comes when the thin stuff ends up cutting into your hands, or not being as comfortable to handle when you go to make an adjustment, so you end up making fewer adjustments than might be desireable. Look at the other boats in your fleet, talk to some of the guys, talk to the guys at SailNet, and then pick out what works for you. Maybe even start the season with the old
lines so you can determine what needs attention from actually using it.