One big difference is that the attempts to build "real communism" have lead to tens of millions of deaths while the attempts to build "real capitalism" has lifted tens of millions of people out of abject poverty.

That is simply wrong. Firstly, the "attempts" do nothing, and they fail for both models frequently. And both for the same reason: because people are assholes. They are greedy and selfish by nature, and unable to function in a system that requires common sense to work for the well being of the society.
So after the "attempts", the ruthless and greedy begin to take over (shit floats on top, as the saying goes) and the whole thing goes south. Thats the same for capitalism and communism. And capitalism has sure killed a lot more people than comminism, just because it had a couple of thousand years to do so. Also it has been the main reason for poverty and inequality, and still is today.