I have a metal gearknob on my car from a previous owner an it gets HOT HOT HOT! in the sun. Like Magma! So I wouldn't do a metal knob unless you cover it when you leave the car. I did plan on changing it until I saw the crazy price of gear knobs and thought I would rather burn my hand everyday than send close on £60 on a knob.
It is odd though, coz when I looked through them all I didn't thing any of them looked particularly nice, too boy racer. Bit like when you want to tile the bathroom and all you can find is dolphins or greek women with water jugs, when you just want something plain and tasteful.
Quite fancy a plain old ferarri black ball knob.