3core brass vs 2core alum - both are the same price - which is one is the better buy?

 

I believe both are the same thickness, 53mm or so. With that said, go with the aluminum, having less tubes will increase the cooling efficiency of the radiator. Its the same reason the a thinner 38mm single tube is as efficient as a thicker dual tube, which in turn allows the ambient air to pass thru an cool more effectivly.