You are comparing complete new systems
Have each bidder use the same runs and discharge destinations so material differences are not confused with different work scopes. For aluminum, ask for the actual product specification and support details. For steel, identify the coating and care instructions. For copper, establish the intended finish appearance and joining method with the supplier. Put these details next to the installed price rather than accepting an unexplained premium.
Notice next: Request product sheets and a run-by-run layout for each option, with exclusions shown separately.