In terms of the greatest factor - I like to look at it this way…
Government control of business (Communism)
vs
Business control of government (Fascism)
There are dozens of other differences though. Racial theories, welcomed or banned religious practices, cultural work ethics, Old Money vs Peasant Class - class warfare, etc, etc ,etc.
It’s not an easy question to answer in a sentence.
Totally disagree. None of the Nazi senior leadership came from the business or industrial class. They were all from the radical camp. Watch the Goebbels Diaries, narrated by Kenneth Branagh. All the words that Goebbels uses, in condemning the “bourgeoisie” for not being “radical” enough, and wanting to tear down “existing” morality, was not at all about the business or traditional or conservative of anything. They wanted to tear down the traditional structures and install a new radical regime (which is why the Nazis had minimum wage laws, strong (albeit state controlled) trade unions, just as in the Soviet Union, universal healthcare, the strongest gun control laws in the world, and nationalized workers holidays). They were radicals.
Why did Nazis and Communists hate each other? Because that is what the Left does: Hate those who are most similar, but still slightly different, to them. Which is why the Bolsheviks hunted down and killed the Menshiviks. It is why Stalin called all those Communists whom he put on trial “fascists.” It is why various leftist organizations who are not aligned with one another fight like cats and dogs. For the left, there is nothing worse than an apostate.