This thread discusses the cast vs billet vs forged vs extruded upper/lower dillema
Long story short, forged is quite a bit stronger. This is because they start out with harder alloy in forgings. If you're going to mill the whole thing out like in a billet receiver, you're going to want a metal that's soft enough to mill. Granted anything within reason can be milled, but 6061-T6 is what they've chosen for the billet.
And they use the 7075-T6 for forgings. Its quite a bit harder to begin with, then the forging process hardens it even more. The forging process can even align the grains of the metal for best strength if so designed.
All methods involve milling, just some more than others. Forging shapes the material somewhat so less milling is needed. This is where the quality issue comes in where some people think billet is cooler than forgings. Unmilled areas of forged parts can look imperfect. A billet has to be milled everywhere so it will always look as it was designed. Forgings depend on a combination of how it was mashed together and what is chosen to be milled for its look and appearance.
The Vltor MUR I have is very very tight on the barrel and inside the receiver. To get my barrel on without removing metal, I had to have the barrel kept outside in 0F temperature and warm the receiver with a heat gun.