What are the benefits and drawlbacks of the different types lower receivers?  Forged vs machined billet vs cast.

Forged lowers are inexpensive, and high quality.  Cast are slightly cheaper, but of lower quality.  Billet is an expensive alternative, and usually 3 times the price of the forged lower, without 3 times the benefit, which is why they stay a miniscule portion of the market.

Most forged are 7075 T6. Billet ones I have seen are only 6061T6. Cast I have seen are 356 T6.
There are what I consider big differences in the properties/strengths of these. Some use the logic that it does not have to be equal to 7075 T6 to function ok. I have always figured why buy what I think is a weaker material lower when stronger ones are available. Funny thing is the billet one often cost more money that the forged ones. The only billet one I know of made from a billet with similar properties as 7075T6 is the newer Diamond series lower by SOCOM. I was told they are made from Fortal aluminum and I believe the  serial numbers starts with a 1xxx. Here is a link to a site that shows the differences in the materials.
www.matweb.com/search/SearchSubcat.asp