Re: New mainsheet - recommendations
1) Samson XLS, Samson LS, or NE Ropes Sta-Set are all good options. I would avoid Cajun/Novabraid/Novatech XLE which has about double the stretch. That's not the end of the world in this application, but Samson LS is about the same price and better line.
2) Do you use the mainsheet to lower your mast? If not 50' is a good length for the mainsheet on a Catalina 25 with a 4:1 tackle. The factory recommends a longer mainsheet (I think it is 75'), but that gives you enough extra to use the mainsheet to lower the mast, using the boom as a gin pole.
3) I have no idea. They do generally cost slightly more.
boats that I sail on regularly: Pearson 28-2, Rondar 505, Yankee 30, Blanchard Junior