Both of these are extremely stable and just as easy to secure. Everything I liked about Debian has been added into Slackware (a decent package manager, certain programs included with GNOME, etc). The main reason I prefer Slack, however, is the community of users. Every issue I've had with Slackware, I could either search for an answer, or post a question at LinuxQuestions.org and get an answer quickly. With Debian, I had a harder time finding answers, so that's why I use Slack
There's my $.02
Jeff |