What is the difference between our 6061-T6 and 7075-T6 lower receiver, other than price? In short, 7075 is a stronger aluminum alloy than 6061. Is this added strength really necessary in a billet lower receiver? That's very debatable. Our 6061 lower receivers are engineered to be extremely strong, and we've yet to see anybody break one. On an AR-15, the lower receiver is not under a lot of stress. The high pressures and forces are all dealt with in the upper receiver assembly. So while 7075-T6 is certainly a stronger alloy, we would suggest that this extra strength is unnecessary. 6061 also has an added advantage of being more resistant to corrosion than 7075.
Here is an analogy that helps put this in perspective. The handle on most toothbrushes is made of plastic. Plastic toothbrush handles don't break under normal circumstances. You could certainly make a toothbrush out of stainless steel because it is a lot stronger than plastic, but what is the point? Stainless steel is a lot more expensive than plastic. The tooth brush isn't going to break either way, so it seems like paying extra for a stainless steel toothbrush is not worth it for most people.
So why do we make a 7075 lower receiver if we don't think the added strength is necessary? Because our customers commonly request it, and our goal is to have happy customers. If you are ok with paying a little extra for the added cost of 7075, you will definitely have a nice lower receiver. In the end, the decision is up to you. With an 80% Arms lower receiver, you can't go wrong either way.
Some more reading here: http://www.ehow.com/info_8078719_difference-between-6061-7075-aluminum.html