Something to consider when comparing Capitalism & Marxism: we've had each of these implemented in two great experiments, haven't we? The US and Russia were both ardent proponents of their respective systems after WWII. And the results have been staggeringly favorable for the country that adopted the capitalist system - a few things to consider: Over the past 80 years, US inflation-adjusted GDP per capita has increased steadily. Early Soviet GDP performed well, and in some cases the growth rate even overtook the US, but eventually stagnated due to military overspending, rampant corruption, and a workforce that was ill-motivated and badly trained. How much of the blame one assigns to each of these reasons will provide a sense of why the system failed - personally, I like to think each played a significant role. Which are directly caused by a state-controlled economy? Surely the workforce motivation, maybe the corruption was exacerbated by the quota system, maybe not. Probably not military overspending. A bit of philosophy: doesn't it make sense, that if we assume people are greedy, that a system which exploits this negative quality might outperform than one that suppresses it? And what of Adam Smith's "universal wealth" notion - that even though Capitalism brings stratification, the general level of wealth in a nation still increases enough to bring positive change to all inhabitants?
There were more factors at play than just the system of government in place. The United States is a moderate region where agriculture can go on throughout most of the year; the Soviet Union, by contrast, was much further North, and much colder. You couldn't grow as much food because there wasn't enough sun and heat. Another issue is shipping. Moving things by sea costs about 1/10 of what it does to move things by land, so having a lot of coastal areas improves commerce, and the United States has lots of coastal area, as well as the Mississippi river, so there is lots of land with access to cheap shipping. The climate and access to transportation weren't the only factors, but what I'm trying to point out is that there was a lot more involved that determined the success of the Soviet Union than just the political system.