In steel, yes, forging a part imparts favourable properties. When you forge a carbon-steel part, you work harden the outer surface and align the grain structure to the shape of the die. You can then also change the properties using hardening or annealing techniques.
Aluminum behaves differently in that it will not achieve the same work hardening surface properties being a soft metal (usually an alloy) that does not heat treat well. Forging will force the grain structure to conform to a forging die which is more beneficial in a lower receiver. The surface "toughness" of an Ar is a function of anodizing, not the manufacture method. Still, if both a forged and milled from billet receiver were made dimensionally identical the forging might be a little bit stronger in some ways - but this almost never happens, billet receivers are usually volumetrically larger, thicker walls, etc. with more robust geometry from a stress standpoint. In an upper, the grain structure of the billet is usually parallel to the bore and there is theoretically some advantage to having all grain run in the direction of the force of the fired round, but as I mentioned, this would be VERY difficult to prove. I've never had an issue with forged uppers and never saw a billet upper definitively shoot better than a forged one personally (all else being equal).
Aesthetics are another matter - billet parts can be made differently from the standard pattern, which some people pay extra for. The best of both worlds can be found in the new VLTOR MUR receivers. They forge them to approximate shape then completely machine every surface. In theory you get all the advantages of a forged part with the wall thickness and aesthetics and uniformity of all dimensions associated with a billet part.
As a general rule, however, AR receivers are usually forged because it makes receivers faster to manufacture and the forging mills sell raw forgings cheaper than the added tooling cost of machining from billet.
A bit long winded, so sorry to the OP for the hijack.