Link: http://adteputek.com/b2evolution/blog2.php/2009/03/28/what-works-better-for-injection-mold

Silicone Rubber has better temperature resistance, therefore it will not harden or loose it properties as quickly as Neoprene. Obviously the price of Silicone is higher than neoprene rubber, but in an application where temperature is a factor, it will cost less in the long run.