It's mostly cosmetically what you want.
Billet's look more modern, but functionally are the same.
The forged are just as strong, or even stronger than the billet due to the high pressures used in forging the blank, then heat treating to lign up the molecules.
Just stay away from cast lowers. CAST IS JUNK! They are inferior in strength to billet and forged.
As far as forgings go, there are only a couple of companies in the U.S. that produce raw forgings to all of the AR manufacturers.
I know this for a fact. It's all about the accuracy of the machining of the manufacturer after that. The CNC ops programs are sold to manufacturers so most of them are the same. SOme may tweek the program a bit to make them unique to others, or to add ambi controls.
Most mag wells are EDM cut for smoothness, but other s have worked out the broaching which is cheaper.