I think comparing Stalinism to capitalism is comparing apples to oranges. For one thing, there are as many "capitalisms" as there are countries. As Doug correctly points out, Stalinism is very time and place specific system, so the comparison to "capitalism" has to be more time and place specific.
But even more important point, often missed in the Western discourse is what in empirical science is called separation of effects. Things as we see them usually have multiple causes and the role of the analysts is to examine the effects of specific causes. This is usually done by control conditions that is, varying one condition while holding everything else equal.
For example, employment compensation can be affected by several factors, such as industry, geographical region, skill level or personal characteristics. It makes no sense to compare individual A and B and say that their earning are attributable solely to their personal characteristics, such as race or gender - we also need to "control" (or hold constant) for all other relevant factors.
This is really Research Methods 101, but when it comes to discussing countries -it is all but forgotten. Any observable difference between countries are attributed to a single factor of person or political choice, capitalism, Stalinism, work ethics, collective IQ etc. One of the reason is that when it comes to countries, "controlling" or holding constant is very difficult, because it is almost impossible to find other cases (countries) that vary only in one selected aspect but are otherwise "constant" i.e. identical. But far more important determinant is ideology - country comparisons are usually made for ideological purposes to show the superiority or inferiority of a particular regime - and in such cases ideology as rule trumps reason.
So to evaluate the effect of Stalinism on Russia, one would need to compare Russia under Stalin to a "counterfactual" i.e. an "alternative" Russia with no Stalin - which is very difficult for obvious reasons. My hunch is however, that is such a comparison were ever made in a rational way, it would show that many bad things attributed to Stalin or "communism" in general would have happened in Russia under any government, i.e. even if the Kerensky government or even some other kind of government stayed in power.
Wojtek