The metal for the clevis depends on which tow bar you are using, not the base plate.
If you are using the Ready Brute or Ready Brute Elite, you will need aluminum.
If you are using the Ready Brute II or Ready Brute Elite II, you will need steel.
The only difference for base plates is the brand. If it is Blue Ox, choose Blue Ox. If it is Demco Tabless, choose that.