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)
I can't really comment on any of that since I've read nothing on the subject so I'll take your word on it.
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I'd say your speculation and opinions are based off of your own studies in that area, so it would stand on much firmer ground than any assertion I could make on the PRC's economic history.