Note here that the difference is between the deaths of currently-living people, and preventing the births of potential people. In hedonic utilitarian terms it's the same, but you can have other utilitarian schemes (ex. choice utilitarianism as I commented above) where death either has an inherent negative value, or violates the person's preferences against dying.

BTW note that even if you draw no distinction, your thought experiment doesn't necessarily prove the Repugnant Conclusion. The third option is to say that because the Repugnant Conclusion is false, it must be that the automatic response to your thought experiment is incorrect, i.e. that it's OK to wipe out a googolplex galaxies full of people with lives barely worth living to save 10,000 people. Although I feel like most people, if they rejected the killing/preventing birth distinction, would go with the Repugnant Conclusion over that.