Acrylic and Delrin are not composites, its all one material through and through. Acrylic is polymethyl methacrylate, and Delrin is acetal homopolymer, a crystalline plastic which offers an excellent balance of properties that bridge the gap between metals and plastics.
Clearaudio used to use acrylic, but has now switched to the use of POM for platters for many TT models, as it is harder than acrylic. POM is Polyoxymethylene, and has properties very similar to Delrin.
On the whole, my experience is that platters made of harder materials tend to sound better than those made of softer materials. Delrin, POM, glass, aluminum, and ceramic are all harder than acrylic. But, its part of a system, so there can always be exceptions.
As for mats, that is very highly dependent on the turntable. Its important to remember that turntables are designed by engineers to function as a system. In the case of many Rega models, the felt mat is required to obtain maximal system functionality. So, if it comes a felt mat, my recommendation is to use it with the felt mat it comes with. I tried cork on my Regas, and did not like it, it made the presentation sound dead. Turns out but all my Regas sounded considerably better when the OEM mat was used on the stock platter (and yes, I tried some aftermarket platters).
