Hi Chris
The standard knob is mostly made of hard plastic including the pressure surface that touches with the friction washers
The Radiation knob is all aluminium.
The SOM is mostly aluminium with the pressure surface area made of a some type of rubber (high temp I presume).
Yes, the SOM is my pick as drag pressure increases in a much more progressive manner than the Radiation. ( I can't remember which site, but this subject of the Radiation has been discussed previously). Note that Plat.co.jp are discontinuing to stock them.
Good luck
Les