Both are major industrial superpower. The third Reich had no chance against one of them, let alone two.

However, the war on the estern front lasted almost 4 years - 4 years of massive engagement of the USSR. 3 out of 4 german casualties occured on the eastern front. So it is safe to say that the USSR was decisive against Germany, even if it would not have been able to survive 1941 if UK had given up.

But the war also occurred in the pacific. Almost all of japanese casualties were inflicted by the americans. The USA were absolutely decivise here.

So, we can't say that one ally was more decisive than another against the Axis.