Why not install the 60 amp breaker now if that is the case? Especially if is three pole or a 277/480 volt breaker. Your double pole plug on breakers 60 amp and below are usually pretty inexpensive and there is a possibility you put in a 40 now, though I'd still normally put in the 60 now if that was the reason.It might also be a future consideration; ie 40 A now with a possibility of bumping to 60A later without having to pull new conductors.
Either conductor size is code compliant I would use the #8 unless there is a voltage drop concern.Have 120/208 3P 4W 100A being fed from 3P 40A breaker. One drawing asking for 4#6 & another drawing asking for 4#8’s. Conductors should be sized off 40A breakers, correct? So #8’s ok.