All things being equal (i.e. the physical dimensions of the rod, material type, etc.) a forging will ALWAYS be stronger than a piece made froma a "billet", or from casting.
The forging process changes the internal grain structure of the piece. These "grain lines" follow the shape of the piece, making it strong.
When you cut a rod from a "billet" (a solid block of material), the grain lines in the piece all run parallel to each other. The machining process cuts through these grain lines, thus making a weaker piece (again, all things remaining the same).
Here is an example of grain structure in a forged piece, and how forging vs. machining cuts the grain lines. (Couldn't find a pic of a con-rod, but you get the idea):
The forging process DOES leave a rough suface finish, which can create "stress risers" and cause a surface-stress fracture. Shot-peening and polishing of the rod's outer surface removes most of this, which further increases the strength. Heat treating is last step in creating a hi-strength forged piece.
The problem with forging is that it is expensive, requiring dies, large presses and big heating equipment.
Cutting a piece from a solid billet is relatively inexspensive, particularly with todays modern cad-cam milling machines: throw the piece in and let the machine run it's program while you sit back and read the paper (well, maybe not that simple, lol...but you know...)
Lastly, "forged billet" is kind of a joke, IMO. Yes, you can have a billet block that has been forged instead of the more conventional roll-forming, but the machining of the piece will still cut through the grain structure of the forging, so there is little advantage to this...other than it sounds cool...