Good question, but here we go with a hearty "it depends"!   

When I developed the stainless steel reed I was going for durability and reliability.  The plastic reeds yield higher rpm and easier starting, but rapidly degrade due to post run heating of the reed seat.  Berrilium copper tends to fatigue and can sometimes be corroded by the fuel when the engine sits for a long time.  I tested a variety of materials and thicknesses and chose the SST reeds as best overall.  Thicker and thinner materials cost rpm (who knows why!)

I have never had any luck with the teflon reeds.

 

         Andrew, I would suggest that after installing a reed valve that you check its sealing ability. I'm also assuming here that your using older Bee style engines with the circlip that holds the reed within the tank. This simple test will work using the new style as well but I prefer the circlip retainer. I've found that some of the plastic reeds had some degree of distortion to them. I install the reed then place a small piece of tubing on the intake venturi of the tank. While pushing on the syringe the air should pass right by the reed but when you try to withdraw the syringe, it should just stop and prevent you from withdrawing. If it is still letting even the slightest bit of air pass it's not working correctly. I've taken the reed out of the tank body, flipped it over reinstalled the clip and it's worked just fine. One thing to also take note is when installing the reed is that your only installing one and not two due to one sticking to the other. I use a very small screwdriver to rotate the circlip within it's groove to make sure that it's seated correctly. I rotate the reed to make sure it is free as well. I had two of the white teflon reeds stick together on me and the clip didn't seat correctly. This is why I suggested my rotating method above, learn by your mistakes. The clip popped out and came through the side of the crankcase shedding all kinds of metal into the cylinder as well basically ruining what was my best running Black Widow. Ken