Which mainsheet to use?  Sometimes it can be a bit like what rig should I use today – is it a full rig day or a radial day?  Do I want a thin mainsheet or a thicker one – or should I standardise on just one?  Mainsheet length is another variable/decision to make.  If it is very short – say 12.5m – the positives are that if you accidentially drop the mainsheet in 20 knots on a run it doesn’t go too far out and you have time to recover before you death roll.  Also you have less in the cockpit. On the downside you can’t go too far by the lee.  The ideal length is probably between 13 to 14m depending on how you tie the mainsheet off in the cockpit. 

Not sure which mainsheet to get? Maybe this article comparing the Rooster and Marlow mainsheets might help.