Well, this is a tough one to answer. Of course the answer you'll get will have a major element of personal opinion. That's how it is when you let people analize situations and events in social science - there's no clear answer, and it all depends on your point of view. You will fit the data you get to your own beliefs.
In social science you have lots of contradicting conclusions even on the same data, and of course you can find exactly the data you need to prove your point, if you look for it hard enough. That's why I would'nt take for granted any decisive or absolute answer to your question.
You can also be sure that the answer to your question is complex and has a little bit of both explanations, mixed with others too.
However, it is my belief that the economic system contributed significantly to the bankrupcy of the soviet state, due to its failing to understand human nature, and what drives people doing what they do. But again , that's coming from a free market supporter, that takes all the historical and economic facts. and uses them according to his outlook.