6061 or 7075? 2024. It's the fatigue resistant aluminum alloy. They use 7075 for the top part of airplane wings (in compression) and 2024 for the bottom (in tension, fatigue prone). Use 6061 where fatigue or strength isnt an issue, 7075 where strength is an issue but not fatigue, and 2024 where ultimate strength matters less and fatigue matters more.
If you were replacing your parts semi-regularly, design a highly efficient 7075 unit. If you're gonna be riding daily and not inspecting for corrosion cracks regularly, use 2024. Check out other tempers as well. T6 isnt the only commonly available stuff. The temper will give you high strength, but will make the metal more brittle, and vice versa.
Credentials (my balls are big, etc, etc): Mechanical engineering grad, I now design satellites, recently finished an aircraft structures training course