You must also consider the length of the wire and the operating or environmental temperature when selecting the size of the wire. If you do use the aluminum wire be sure to use a compatible receptacle and apply the corrosion inhibitor at the connection. Number 8 copper is good for 40-55 amps depending on the type selected based on the insulation and application but the same gauge in AL is only good for 30-45 amps. Number 6 copper will safely carry 55-75 amps in a heat range up to 140-190 deg. F. If you are not sure it’s best to step up the wire size and unless you really are sure how to properly install aluminum wire it’s safer to stay with the copper. You pay more for it but it is safer than a corroded connection starting a fire. For reference you can get a copy of the National Electrical Code book at your local library. Another thing is to make sure the ground lead matches or exceeds the size of the feeds.
Wayne