I don't claim to be an expert, but my understanding is that Alpine is mainly focused on small size, so musl libc, lack of systemd, etc, are means to that end.

Void seems to be more focused on power user experience, in the vein of Gentoo / Arch. So glibc or musl as an option, lack of systemd, and a pacman-like package manager (xbps) are all means to that end.

Alpine gets lots of usage as a Docker base layer due to that small size. Void certainly could be used as a Docker base image, but that's much less common than Alpine because the image sizes are probably roughly comparable to Debian and when running in a container the systemd vs not-systemd choice becomes less relevant.

Void: Rolling release, musl and glibc variants.

Alpine: Stable releases that are supported 2 years, musl-only. EDIT: Oops, forgot that "edge" is a rolling release.

I personally prefer Void for desktop and Alpine for servers/containers/appliances, but either could be used for either.