Firstly, never, ever buy a cast lower or upper. It is the weakest form of the 3 metal manufacturing processes.
Seekins Precision billet lower: It is an awesome mix of beauty in design and improved function
However, the difference between billet and forged is often talked about, argued about and speculated on. I have read many a convincing argument for either side on many a forum before I realized I needed to do the research myself. Rather than look specifically at firearms, I researched metals and the manufacturing process and I learned a few things, first and foremost why there is so much controversy. In a way, everyone is right:
VLTOR Lower with enlarged mag release
Forged or billet, an upper or lower made of 7075 aluminum is not going to fail you. Neither is 6061 for that matter, though it is a lesser alloy strength wise. Ounce for ounce, forged is stronger. There is no doubt on this. Properly made, forging compresses the aluminum structure and creates grain lines in the metal that improve strength. Billet parts, while weaker ounce for ounce, often have reinforced areas, making them stronger overall and just a tiny bit heavier when compared to the overall weapon weight. If you want an explanation on the Billet machining process you can click the link.
So the question is what is it you want from the lower or upper? Both withstand more than the type of abuse they should ever be subjected to and for most applications. On that basis and that basis alone, forged makes more sense as it is cheaper and stronger. But guns should be for life. If you are buying something that will last a lifetime, look and feel is also important. For me, buying a billet lower is worth it if it is not too much more money. Some, like Mega Arms, VLTOR and POF (to name a few) have improved functionality also, like a built in tension screw to the upper, improved mag release and flared magwell respectively.
Simply put if you want a good look or new function then take advantage of the billet options to find something that is just right. Otherwise try and stick with 7075 forged aluminum if strength, weight and price are your main requirements.