To some extent, the answer to this depends on your perspective on international relations. One school of international relations holds that nothing in international relations is about ideology. Instead, it is all about power. From that point of view, the difficulties between the two were not about Marxism but rather about power. They were about who should control various areas on their shared border and about who should be the leader of global communism.
However, many people think that ideology matters. In the case of China and the USSR, it is said to matter because China and the Soviets disagreed over how to relate to the West. Khrushchev tried, for example, to coexist peacefully with the West while Mao wanted confrontation and aggressive backing of revolutions around the world. There were also conflicts between the two countries about how best to build communism within their own countries, with the Soviets being annoyed when China did not follow their lead.
Overall, though, scholars can disagree about how important ideology was as compared to how important power was. There is no one "right" answer agreed to by all.