I didn't say better. Cu is obveously a better material for blocks due to its thermal properties.
But Al is much easier to work with. This makes it especialy well suited for a newbie who is just starting to learn how to machine. It is also better, IMO, for prototypes. If design A is better than design B, it will be better no matter what metal you choose to use. With Al, you get faster turn arround with your prototype and it is cheaper, so making multiple prototypes wont hit your pocket book as hard.
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