Which aluminum alloy is best for AR-15 lowers - 6065 or 7075?

6065 or 7075 - which aircraft-grade forged aluminum alloy is best for AR-15 lowers? Selecting a grade is one's task on finding a balance between performance and cost. 
A good place to start is by understanding similarities and differences between 6065 and 7075 aluminum alloy. 

Similarities:
The two alloys are very similar. They have the same agents such as magnesium, silicon, copper, iron, zinc, titanium, manganese, etc. Both are heat treatable. 
Differences:

The real differences are found in the amount of each agent present. For example, the 6065 aluminum alloy contains less zinc than the 7075 alloy. Thus, the 6065 alloy provides better welding and work-ability than its 7075 counterpart. However, 7075 aluminum alloy offers superior strength over the 6065 one.
There is a lot of stress and pressure created when firing your AR-15 rifle. AR-15, however, is a smaller round than AR-10 for example. Thus, when milling the lower for AR-15, 6065 aircraft-grade aluminum provides a great flexibility, enough stress relieve and durability. And it costs much less than its 7075 counterpart. You can find Moriarti’s AR-15 80% lowers at low competitive prices which you can count on to be extremely durable and reliable.
Don’t forget to take a look at Moriarti’s collection of AR-15 Complete Upper Receivers that make a perfect match for your new build!