I just installed a new parts kit (gaskets, replacement needle, fuel screen) in to my 3W-106 tillotson carb. The original flapper valves look to be made of a transparent mylar looking material, while the replacement is more like a black neoprene rubber type.

What is the prefered material here?

As far as I know, the mylar looking one will withstand alcohol and nitro methananol mixes where the black rubber like one was for use with gasoline/oil mix.

I personally prefer the mylar looking ones as they are seem to be able to stand any kind of fuel type.