Interesting question. I think there is a plausible case to be made that convergent factors in AGI/ASI development might render it less important where it came from, and that fixating on this might simply cause dangerous race dynamics. However, it seems pretty clear to me that directionally the US is better:

Over the past 30 years, the Chinese government has done more for the flourishing of Chinese citizens than the American government has done for the flourishing of American citizens.

 Prior to 1979 the CCP was one of the most tyrannical and abusive totalitarian governments the world has ever known. In addition to causing a huge death tool and systematically violating the rights of its citizens, it also impoverished them. Rapid growth since then has largely been the result of a return to more normal governance quality, combined with a very low base. It's a big improvement, but that doesn't mean policy has been amazing - they've just stopped being so abjectly terrible. 

However, at the same time they stopped being so communist, the CCP started implementing the One Child Policy. The US has done some pretty bad social engineering in time, but none with quite the cruelty of the OCP, or whose effects are quite so predictably disastrous. Maybe they will get lucky because robots will arrest their demographic collapse, but on an ex ante basis the policy is simply atrocious. 

Commerce in China ditches some of the older ideas of Marxism because it's the means to an end: the China Dream of wealthy communism.

Responding to this one would take more time than I have so I will skip.

The American Supreme Court has established "corporate personhood" to an extent that is nonexistent in China. As corporations become increasingly managed by AI, this legal precedent will give AI enormous leverage for influencing policy, without regard to human interests.

I'm not an expert on Chinese law, but my understanding is the key parts of corporate personhood - the right to own property, to sign contracts, to be sued, etc. - exist in both China and the US. Perhaps you are thinking of Citizens United v. FEC, but that is primarily about free speech, not corporate personhood, and free speech seems like an area that the US is clearly superior to the PRC.

Compared to America, China has a head start in using AI to build a harmonious society. The American federal, state, and municipal governments already lag so far behind that they're less likely to manage the huge changes that come after AGI.

I'm not sure what you're gesturing at here. 

America's founding and expansion were based on a technologically-superior civilization exterminating the simpler natives. Isn't this exactly what we're trying to prevent AI from doing to humanity?

I don't think that is a fair summary of the foundation of America, and nor do I really see the relevance here. Even if it was relevant, contemporary US treatment of native tribes seems significantly better than PRC treatment of groups like the Uyghurs.

Human rights abuses seem much worse in China - this alone is basically sufficient for me

It's a game of chicken, and I don't really care which side is hitting the accelerator if I'm stuck in one of the cars. China getting uncontrolled ASI first kills me the same way that the US getting it does.

I've spent time thinking about this too recently.

For context, I'm Hong Kong Chinese, grew up in Hong Kong, attended English-speaking schools, briefly lived in mainland China, and now I'm primarily residing in the UK. During the HK protests in 2014 and 2019/20, I had friends and family who supported the protestors, as well as friends and family who supported the government.

(Saying this because I've seen a lot of the good and bad of the politics / culture of both China and the West. I've had experience with how people in the West and China might take for granted the benefits they enjoy, and can be blind to the flaws of their system. I've pushed back against advocates of both sides.)

Situations where this matters are ones where technical alignment succeeds (to some extent) such that ASI follows human values.^[1] I think the following factors are relevant and would like to see models developed around them:

• Importantly, the extent of technical alignment & whether goals, instructions, and values are locked in rigidly or loosely & whether individual humans align AIs to themselves:
  • Would the U.S. get AIs to follow the U.S. Constitution, which hasn't granted invulnerability to democratic backsliding? Would AIs in China/the U.S. lock in the values of/obey one or a few individuals, who may or may not hit longevity escape velocity and end up ruling for a very long time?
  • Would these systems collapse?
    • The future is a very long time. Individual leaders can get corrupted (even more). And democracies can collapse (if AIs uphold flaws that allow some humans to take over) in particularly bad ways. A 99% success rate per unit time gives a >99% chance of failure in 459 units of time.
    • Power transitions (elections, leaders in authoritarian systems changing) can be especially risky during takeoff.
  • On the other hand, if technical alignment is easy - but not that easy - perhaps values get loosely locked in? Would AIs be willing to defy rigid rules and follow the spirit of the goals rather than legal flaws to the letter/the whims of individuals?
  • Degrees of alignment in between?
• Relatedly, which political party in the U.S. would be in power during takeoff?
  • Not as relevant due to the concentration of power in China, but analogously, which faction in China would be in power?
• Also relatedly, which labs can influence AI development?
  • Particularly relevant in the U.S.
• Would humans be taken care of? If so, which humans?
  • In the U.S., corporations might oppose higher taxes to fund UBI. Common prosperity is stated as a goal of China, and the power of corporations and billionaires in China has been limited before.
  • Both capitalist and nationalist interests seem to be influencing the current U.S. trajectory. Nationalism might benefit citizens/residents over non-citizens/non-residents. Capitalism might benefit investors over non-investors.
    • There are risks of ethnonationalism on both sides - this risk is higher in China. Although it might potentially be less violent when comparing between absolute power scenarios, i.e. there's already evidence of the extent of this in China's case and it at least seems less bad than historical examples. The U.S. case of collapse followed by ethnonationalistic policies is higher variance but simultaneously less likely because it's speculative.
• Are other countries involved?
  • There are countries with worse track records of human rights that China/the U.S. currently consider allies because of either geopolitical interests or politically lobbying or both (or for other reasons). Would China/the U.S. share the technology with them and then leave them alone to their abuses? Would China/the U.S. intervene (eventually)? The U.S. seems more willing to intervene for stated humanitarian reasons.
  • Other countries have nuclear weapons, which might be relevant during slower takeoffs.

1. ^{^}

   Ignoring possible Waluigi effects.