Also, which side contributed more? Traditional Chinese school of thought has it that the Communist forces tied down a large body of Japanese troops with guerrilla warfare, but the large engagements were all fought by the Nationalist forces. Is there any evidence for it? Which side was more effective?
14 million Chinese deaths during WWII. (Contribution).
Which side "contributed more"? I assume KMT vs. CCP.
That's a very complex question.
1st, it was well known that KMT under Chiang Kai-shek wanted to avoid fighting Japan, in order to preserve his forces to annihilate CCP forces. Part of this was that Chiang didn't think he should openly fight large battles against the Japanese army, who were better equipped than the KMT forces.
But, KMT was held most of the large cities in China (connected by roads), and they became the obvious target for the Japanese army.
So, time after time, KMT forces were forced to fight the Japanese army that was marching through China, in bloody battles, with significant losses to the KMT forces.
2nd, CCP retreated into the North, specifically with guerrilla warfare in mind.
This meant that they would harass Japanese and KMT forces alike, but avoided open pitch battles.
Undoubtedly, the CCP tactics were more effective, because precisely they avoided confronting main Japanese military forces.
Interestingly, via some coordination with USSR, Mongolian Communist Partisans were extremely effective in harassing the Japanese occupation forces in Manchuria, which effectively shielded the CCP from large scale Japanese incursions into CCP held territories.
It has been my assessment that the KMT was focused mainly on eliminating the CCP during the war and was only interested in the Japanese as a side issue. That is not to say they didn't fight, just the focus was on the CCP.
Both sides saw the Japanese as foreigners and there was a strong push-back.