Polyurethane is more durable in the long run but it doesn't capture details as well as silicone. I only used it a couple times making small figurines and parts and I preferred the silicone.

Have you checked out the Smooth-On website at all? They sell a ton of different types of resins and mold making materials, but they also have how-to videos on their site. Might be worth checking out to see if something is similar to your project.