Wow what a pack of misinformation!. The "hardness" of the material has nothing to do with it's method of manufacture but it's alloy and it's heat treat. Forging will not increase the density of a metal either. Forging has this benefit- it help align grain structure in the metal to increase it's tensile strength IF properly heat treated. This is primarily due to grain structure control. Non heat treated forgings indeed are often of higher tensile strength than castings of the same alloy. A proper heat treat process will make the difference negligible in any non load bearing or structural application. It is more expensive and if paying more for something means it's "better" to you, go for it.
Often when heat treating fully machined parts they warp as the billet had stress which when machined to shape were released and the part warps.
As the upper and lower of an AR are not load/stress bearing structural parts the difference is final appearance to you and cost when any of the 3 methods is properly done.