Aluminum moulds are more succeptable to galling at the pins and wear on top from the sprue plate, so they generally require a little more maintenance and lube.
That said, most of mine are aluminum simply because they are lighter and easier to work with that way and thats what the makers choose to extend the life of their tools.
If weight isnt an object, brass is my favorite because it maintains heat well, doesnt need oiled or stored "seasoned" and can be made on the same equipment as aluminum, which makes for a bit of design flexibility.
It isnt as "soft" as aluminum, so it doesnt need as frequent PM.
