RE: RDM VS. SDM
The best thing to do is to look at what kind of mast the sail is designed for. For exsample I'm using RDM masts on all my hot sails maui sails, but on the NP expression i use a SDM, no matter the sail size.
The most important thing about the mast is - for me - that it has a lot of carbon in it (at least 75%). Although a C100% RDM is less likely to snap than a C100% SDM mast because it has thicker ''walls''.