Disregarding copper because of price, which is the better material for heat transfer: Aluminum or brass? From the research I've done, including contributions from this site and the "other site," it seems aluminum wins in this category. I wonder why brass is so popular here then. Why not just stick with aluminum since the reflector and flashlight body are also made out of it?
Because brass is easier to solder (alloy is pretty much not to solder), so there ist a brass ring needed (press fit to the alloy pill) to hold the driver.
ahhhh... I see. I was debating between the brass or aluminum pill but if I won't see a significant increase in heat transfer anyway then maybe I won't need to bother.