I agree "stay away from polymer". That said, some people like them and swear by them.
In aluminum, just go with one of the companies that gets good reviews online and you should be good.
Here's a short list, not all encompassing by any means, of manufacturers I have dealt with.
Aero is good. As mentioned, they manufacture for a lot of others such as Palmetto State. Be careful though, just because they manufacture it doesn't mean it's the same. For instance, the Palmetto lowers have thinner walls making them a bit lighter. They also lack the upper tension screw that is standard on Aero's own lower.
Anderson make a decent product. I have had some issues with anodizing on them but the ones I have blasted and Cerakoted have held up fine.
I've had good luck with Double Star. Some people like the finish they put over anodizing.
I like, and sell a lot of Black Creek Precision lowers. Similar in weight and cut to Palmetto but has the upper tension acrew. They also thread the hole for the bolt release pin and give you a dog leg screw to replace the pin. It's a little easier for first time / inexperienced builders to do without marring the finish. Lastly, but most importantly in my mind, they thread the rear take down detent hole and give you a set screw to retain the spring so you don't have to fight it when installing the buttstock.