Hi!
Me, for steel challenge - and the lighter loads you're going to shoot - I'd stick with polymer. They're surprisingly tough and you keep that gun lighter for less momentum swinging plate to plate.
C-more came out with the aluminum versions when they started selling the Railway to cops for AR-15s that got tossed into the trunks of squad cars so they needed something a little tougher.
I doubt your steel challenge gun is going to be abused in the same manner.
Click is definitely the way to go over the rheostat version; it may be more expensive, but you'll have an easy, positive, repeatable brightness setting with the defined positions of the switch.
Aluminum models only come with the click switch.
Thanks for the question!
Sean.