What size and type wire should be used for 100 ft away work shop

I built a shed a while back and. now I'm ready to get power to it.

I have a 50 amp GCFI breaker, that I plan to run the service line from to the shed. The shed is roughly 100ft away. I called a supplier and they recommended I go with 6 gauge copper or 4 gauge aluminum.

I'm curious which would be the better option?

To cavitate, I prefer the cheaper route as long as it is suitable for the application. The Aluminum 4 gauge is half the price of the 6 gauge copper, but both are reasonable.

Also, I plan on digging an 18-inch trench and running wire through PVC conduit.