Go with 7075. It is twice as strong (503 vs 276 MPa Yield strength). It is the same stiffness, (71.0 vs 68.3 GPa). It has a poor rating for workability (bending), but I have made chassis with 25 degree kickups that have had no fatigue or cracking problems. Just a reality check on the strength: it is harder to bend. It's quite dramatic in comparison to 6061.
7075 is the strongest aluminum that is commercially available (easily). I have milled it and also cut it with a jigsaw. Either will work. You will notice the increased force when cutting with hand power tools, but it's not a big deal.
I'm not sure where the idea comes from that hardness is equivalent to stiffness (it isn't; hardness is analogous to strength), but it acts like 6061 until it's time for it to fail (scratches, plastic deformation). Then it is vastly superior.
7075 costs about twice as much as 6061, and it's about twice as strong.
References:
https://online.kaiseraluminum.com/de..._and_Plate.pdf
https://online.kaiseraluminum.com/de..._and_Plate.pdf