GO ALUMINUM! Were the plastic available when I was building? I'm not sure but probably. Never gave them a thought. These are things you want to get the best of as you will be using them
EVERY time you fly. There is nothing like having the vents directed precisely where you want them with exactly how much air you want blowing out of them. Sometimes even a small adjustment makes a major difference. Imagine the annoyance when a plastic one breaks or is leaking while you are flying and venting unwanted air on your passenger who wasn't sure he wanted to get in this airplane in the first place!
Oh, great! The thing is falling apart! And they are a PITA to replace.
I have two 1-inch aluminum vents on the panel, one on each side, which I can direct exactly where I want them. They keep me cool in a canopied RV-4 without the use of a
shade.
I have never been "too hot", and I fly with a helmet! The 1 1/2" vent for the passenger takes air from the bottom of the starboard wing via a NACA scoop and will literally blow your hat off!
I have both the vent itself and a divider gate which can further regulate how much air is coming in and also mix with warm air when needed, or completely shut incoming air off. Never had a passenger complain about the temperature.
Aluminum vents speak quality. "You can pay me now or you can pay me later...." IMHO; those that like their plastic ones: here's to ya!