I like both versions: Yes, the one with Kevin sounds more balanced, but I also like the contrast between your and Solaria's voices in the harmonies, they give the song something the other version doesn't have.

In general, human voices take a lot more effort to sit properly in a mix than vocaloids. A little compression to keep your natural voice levels more even, some reverb, and adjusting the voice levels (not in general, but where they need to be louder or softer) and you're there.