What were the material differences such that the USSR was able to use central planning to develop their economy into a superpower, while the Chinese economy struggled to alleviate poverty without seeking foreign investment?
(I get that the question presupposes legitimacy of the Deng reforms and I am too much of an ignoramus to argue one or the other, so if you think the question is a fundamentally incorrect one I'm interested to know why too)
It’s a good question and I don’t know enough to answer it articulately. Somebody else will have more useful insights for sure.
My idea is that nothing actually would have prevented China from industrializing without importing western capital, it just made the process faster and less toilsome. The options open to China were essentially industrializing solo, with incredible amounts of toiling, or importing capital from the USSR which began to be less appealing with the Sino-Soviet split, or import it from the west which is what they ended up doing . The USSR never really had an option to do that post WW2 even if they had wanted to, China had that opportunity precisely because the west found it politically acceptable to do business with China as part of winning the Cold War against Russia. China also pursued the first two options to various extents before the Sino-Soviet split and reproachment with the west.