The U.S. easily afforded both guns and butter (although you couldn't get a new toaster or car or tires)
Maybe, but we weren't having our country invaded, either.
The USSR built about 25% more tanks than the USA (and the tanks they built were unarguably better - we didn't have a good match for the T-34 until the M26 Pershing, which barely even saw action in the war), and nearly twice as many artillery pieces and mortars, for example. And that AFTER Stalin's massive purges that wiped out a huge portion of the nation's resources (and how many bright and industrious workers?).
Be cautious when drawing too many conclusions from the USA's apparent relative success in WW2...there were a lot of factors in play.