The much better ballistics and 50% greater hitting power of the 3-gun B-20 package more than make up for 50 fewer rounds per gun with the shVAKs. The only reason to prefer the 2-gun package is for the challenge.That being said, I prefer the La-5FN and it's 2 shVAKs. They can do the job just fine.
Also how do you figure 30lbs? 50 rounds of ammo weighs 26lbs and the two gun package is heavier than the three gun package.