6061 Alloy
Extremely versatile, this heat-treatable alloy combines good weldability and formability, high corrosion resistance, and medium strength. Use it for chemical equipment, vehicle parts, scaffolding, and pipe fittings. Yield strength is 40 ksi. Hardness is 95 Brinell. Melting range is 1080 C to 1205 C.
7075 Alloy
One of the hardest aluminum alloys, this exceptionally strong, heat-treatable alloy has good machinability and fair corrosion resistance. An "aircraft alloy" material, it's ideal for aviation parts, keys, gears, and other high-stress parts. Yield strength is 73 ksi. Hardness is 150 Brinell. Melting range is 890 C to 1175 C
7068 Alloy
Offering extreme high strength, this heat-treatable alloy is significantly stronger than Alloy 7075 with comparable corrosion resistance. Originally developed for ordnance applications, it's now being used for aircraft and vehicle parts as well as for other applications requiring extreme high strength. Yield strength is 99 ksi. Hardness is 190 Brinell. Melting range is 890 C to 1175 C
To answer your question, 7075 is better than 6061 because it is harder and stronger. As you can see, 7068 is better yet, but nobody offers this grade in AR receivers. Interestingly, 7068 is used in Beretta's pistol frames.
BTW, 1 ksi = 1000 psi