They are on par with each other.
Zenwalk provides a more complete xfce desktop out of the box, but Salix seems to have the more extensive repos. I think Zenwalk is a 1-2 man operation these days, and they're putting out a solid distro; Salix (and Slackel) have the larger team, so a bit more polish.
I may switch over to Slackel the next upgrade to the Slackware-current branch (about 3 months) just to have more of a rolling release. I've also thought about installing Slackware-current, then installing gslapt with the Slackel repos.
Back on topic -