If I can prove with mutually agreed facts that capitalism has a higher body count that any other political system, will you agree to change your position.
Usually it's Western Capitalism vs Communist [Dictatorship] Regimes and the argument is entirely valid. If you're forced in to it it's not communism -- people won't in general follow an ideology that contradicts their natural propensity to greed and winning above others (pride), so it generally is not communism when you're talking about groups above a small town size.
I spend quite a bit of time thinking about how to transition from our widely oppressive capitalistic system to a more communistic one in a way to accommodates people's propensity towards greed and still maintain democratic freedoms to the greatest extent. It's something that in the long term might solve a lot of our overuse of resources.
The problem, of course, is that without force they’re short lived, like volatile molecules exposed to air. All matter eventually retreats to its most stable form.
Of course, nothing stops like minded individuals from forming communist societies (communes), and they do sometimes. Even a small one doesn’t stay together long without force or a very strong binding identity, such as religion or family.
Even so, if you agree upon the metric and then use it to rank systems, if one system ranks better then you have "proven", by the agreed upon terms of comparison, that this system is superior. There may be other systems outside the ranking, but your proposal was to rank certain specific systems, not to discover the optimal system.