Firstly, if it’s a prototype, I would absolutely say go with the urethane casting. That manufacturing process is so much better suited to low-volume production than something as costly to set up as injection molding. So definitely stick with the urethane casting until you’re sure that you want to make thousands of units.

Casting will also be great for fine surface details. But you do need to keep a careful watch on dimensional accuracy, as the silicon molds show wear and deformation after as few as 30 units. Is your dimensional accuracy important due to a press fit or watertight seal, or are you purely worried about the surface details? I think as long as you have a small production run (fewer than 20), any inaccuracies should not be noticeable, and casting is still the way to go.