This question basically rolls the Ford vs Chevy and Glock vs 1911 debate into one contentious discussion. What should an AR receiver be made of? Traditionally (mil-spec) lowers are forged from 7075 aluminum. Many consider this the standard by which all other lowers are judged. Forging adds strength and 7075 is the strongest aluminum alloy used in lowers. A step down from that is a machined 7075 billet lower. The 7075 alloy is still strong but since it is not forged it is not as strong. I have personally not seen a 6061 forged lower so we'll skip that possibility and go to a machined 6061 billet lower. Not as strong as either 7075 variant but still tough and much less difficult to machine. Another aluminum based option is the cast A380 lower. The knock against cast aluminum is that is more brittle than billet or forged aluminum. But it is easy to machine and the cost is very competitive.
Finally there are the polymer lowers. I believe Plum Crazy and then/now New Frontier Armory brought these to popularity though the first ones I was aware of were from Cavalry Arms. Due to their ease of machining polymer, 80% lowers have also become popular. The first generation of the EP Armory (also sold by Ares Armor) polymer lowers used a jigless design which allowed anyone with really basic tools (Dremel and some bits) to complete a lower. That is no longer the case with the 2nd generation EP lower as it is all one color. But there are also polymer lowers made to be used with an end mill and a jig such as the James Madison Tactical (JMT) and the new Polymer80 (also sold by Ares Armor) lower. In the polymer lower market there are two types of polymer, reinforced and not. The original EP Armory along with the JMT lower are reinforced polymer. The first generation Polymer80 G150 was not but the new version is. Reinforcing fibers add strength which can also be gained by simply adding more material to the receiver design. In any case a polymer lower is much easier to machine than either aluminum alloy.
So which do you choose? Personally, I have no problem using any of the above listed materials for an AR lower. The AR lower receiver does not incur a lot of stress which is why it can be made from polymer in the first place. You really have to ask yourself what you are comfortable with and how well you understand the material choices. Many people will say you have to use a mil-spec forged 7075 lower or you are inviting trouble. Fine, that's an easy stance to take and if you are a nervous person or truly believe that is the only smart choice then by all means choose that. There is nothing wrong with it and the design is proven. But if you are not as concerned with having the absolute strongest receiver out there, or maybe you want something more stylish, then look into a machined billet lower that has a design and features you want. Whether that is 7075 or 6061 is up to you for the same reasons. Will you worry that your lower is going to break if you don't get 7075? Then get 7075. But just understand it will be slightly more work to machine. Sometimes a design you want only comes in 7075 which makes your choice for you. If you want something really easy to machine or are looking to do a lightweight build then consider a polymer option. Under normal use any material out there should hold up. Now if you think this is something you might hit or abuse then go with one of the stronger options. If you're going to go hunting or otherwise pack it around in the woods durability may be your main concern or maybe even lightweight. The bottom line is you have to decide what is important to you and what concerns you will have if you make one choice over another.
Now let's take a look at the jig we have chosen to use for 80% lower completion, the Easy Jig. Click here