When you give TO a large enough design space, you are simultaneously optimizing the external layout and the inside geometry.

What these two Pareto fronts have shown you is that AlSi10Mg is always superior to the other material, so the material decision can be made now unless there is another constraint like cost or strength.

The Pareto front for one material lets you trade-off Weight vs. Displacement, but at the end of the day, you have to pick an allowable limit for one or both variables to help choose a particular design. If you put allowable limits in the TO as constraints, they will help to create the optimum design.