Rubber would make it very sticky and hard to get the boat off the trailer. Rubber to PVC would also leave marks and possible wear areas. Rollers don't have enough support. In my opinion carpet is best for SIB's. Carpet can lubed so the boat slides on and off easily, and has less chance of causing wear during movement.
As already advised carpeted bunks are eminently suitable for a sib. Forgiving on the soft pvc fabric, super slippy when wet and slippy enough dry.
Sand not usually issue if launching/retrieving off trailer and if launching dry utilising transom wheels then best keep hull clear of sand. Sand will be an issue whatever the bunk material.
