On advice of an RV-9A builder, I went with the aluminum ones too.
I also had to remove the original placement of my vents. But for me, that was because I chose a poor method to clamp them on. So I spent several hours scraping sealant off (I chose to use sealant rather than silicone to better promote paint adhesion).
Here's my description, with pictures, of those trials and tribulations.
However, even though they are the aluminum ones, it's worth noting that my left vent would quickly close, from any position, on its own in the airstream. I had to take spray adhesive
and paint it on the area where the butterfly rotates. It increased the friction quite a bit (so now I need to push on the butterfly to begin to open and to fully close it) yet it will still close in flight, but now over the course of several minutes. My right vent stays open in all positions.