Bike Light

I'm making a bikelight. I was going to use aluminum and braze aluminum tubing to a heatsink, however, I only have acess to steel so today I welded together some steel tubing. I'm wondering, how does the heat conductivity compare with that of aluminum? I'm conisdering using the steel tubing I welded together as a base peice to make a sand mold out of and cast aluminum instead, do you think it is worth it? Or should I just get some copper pipe and solder it together? My design is 2x3 peices of 1" steel pipe and is 1.5" long. I'm going to put a backplate on it and attach it to a heatsink, which I cut in half. It will go like:
heatsink pipe pipe heatsink
heatsink pipe pipe heatsink
heatsink pipe pipe heatsink

http://en.wikipedia.org/wiki/Thermal_conductivity
short table, a little over halfway down with some thermal conductivities (higher is better). (stainless) steel is 12-45 and aluminum is 120-230. copper is over 400.

http://en.wikipedia.org/wiki/List_of_thermal_conductivities
that is a long table.

so it is up to you, depends on how much power to expect to disapate as heat and how much trouble it is to make, get copper