The purpose of using a Billet Crank is not for the increaed strength. It is becuase the crank needs are different than is available in the quality forged pieces.
If you can get what you want in a quality USA Crank that was forged by either Kellog or by the Louisville Foundries, by all means go with the forged one.
Those crank companies that do offer both forged and billet will tell you that they feel the forged is stronger. David Crower of Cower, and Jay of BRC will both tell you they believe their forged cranks to be stronger thn their more costly forged ones using the same 4340 material.
With a forging you will have grainular flow pattern developed inside the crank that will be similar to a grainular flow pattern of a tree.
With a billet you will have the grainular flow patern interrupted by the cutting of the biillet.
Wineberg offers their cranks in either the 4340 material or the EN 30B material. Sonny Bryant offers their billet cranks in the 4340M material. Arguably the 4340M, and the EN30B are more ductile than the 4340. I don't know. Joe Squires at Sonny Bryant says that their alloy is stronger. Randy at Wineberg says they are about the same.
Ed