It seems unlikely that the company really doesn't care either way.
But if that's the case, my answer would depend on my level of interest in the open source project.
If it was something that I could see myself continuing to contribute to in the future even if I was working a different job, I'd prefer to use my personal email address, so there would be continuity in that case.
If it was something that I didn't have any personal interest in and was only contributing to as part of my job, then I'd prefer to use my work email address, so that when I moved on to a different job in the future, I'd break off communications with the project.