I found out first hand which one is better due to a storm ripping one clean off! :B
Emergency rv mobile repair call when pouring raining and I had no vent cover. Mobile guy came right out and installed a new one for me and I was good to go.
'Fast forward 6 months' and I am now in Florida during the hot hot summer, ac roaring trying to keep the coach barely cool.
I am walking thru my MH which has 3 roof vents I get to the middle of my coach and all I feel is a blast of heat on my head and neck. Look up and I now realize that the replacement vent cover the guy put on does not match my others 2. It was almost clear WHITE.
That white vent cover did not a dang thing to keep the heat out. While the other 2 which were smoke colored did.
So yes in my experience the smoke ones are the way to go not ever white. White also lets in a lot of UV's that can discolor your interior also.
BUT. come summer and winter RV'ing you really HAVE to put vent pillow covers in ANY of the covers because they are all just a plastic lid and do NOT seal when closed. Letting heat and cold in. And letting your ac and heat out. Covering them really drops the temp in summer time.