We use both. Silicone is easier to mess with but doesn't last as long before they degrade.
HDPE molds are bullet proof but you need to use some sort of release to get the blanks out easy. You also need to make sure to seal the seams to prevent leakage.
I started out on the cheap, using 5 and a half long pieces of Schedule 80 PVC pipe and a Jerry built jig to seal the bottoms against a piece of HDPE with a silicone gasket. - Dave