When comparing Slackware vs FreeBSD, the Slant community recommends Slackware for most people. In the question“What is the best operating system for a developer?” Slackware is ranked 3rd while FreeBSD is ranked 5th. The most important reason people chose Slackware is:
Uses stable, plain-vanilla packages from upstream.
Specs
Ranked in these QuestionsQuestion Ranking
Pros

Pro Stable
Uses stable, plain-vanilla packages from upstream.
Pro Strong adherence to UNIX Principals
Pro Ideal to learn more of Linux
Follows the original Linux roots. It still sees Linux as a free clone of UNIX so the distribution tries to be UNIX-like.

Pro Package management
Uses standard tar archives with shell scripting as packages.
Pro One more distro which is init based
Some users don't like to install systemd based distros because they increase booting and processing speed.
Pro Oldest surviving Linux distro
Besides Debian (which was first released in August 1993), it is the oldest still maintained Linux Distribution and was first released in July 1993.
Pro Super fast
Pro No systemd
Pro Powerful ports collection
Ports provide a wide collection of software which are easy to build, install and modify. They contain recipes and patches to build various software, so you can simply run "make && make install" to build and install the software. You may also keep local patches and it would be picked up automatically. It also allows you to use the latest software even if you are not using the latest version of the operating system.
Pro Batteries included
Base system contains basic developer tools, including compiler, debugger and system utilities.
Pro Jails mechanism
Jails allow for partitioning a system into several independent "mini-systems", a feature not found in many of the popular OS choices.
Pro Stable updates
Pro Great philosophy
FreeBSD won't change everything from a version to another, it tries to keep and maintain old tools as long as possible, and won't replace half the system every two versions like GNU/Linux distributions such as Ubuntu, Arch or Fedora does. everything is stable and also pretty minimal. FreeBSD is designed for performance.
Cons
Con Not user-friendly
It is made to be KISS (keep it simple, stupid), so you have to do everything by hand.
Con Large size live ISO
One needs to vain 3GB+ data for downloading one slackware ISO.
Con Narrow repos
Doesn't offer the same amount of options as other distros do.
Con Niche/small community
Slackware is its own niche and has a small community.
Con Very slow release cycle
Hasn't updated in 3+ years.
Con Dependency issues
When it comes to dependencies, Slackware shows more issues than many other distros.
Con Pathetic third-party application support
Need to run Photoshop? No luck. Even most software that's portable to Linux will have a hard time running on FreeBSD. You're stuck with ONLY running the free software.
Con Inferior driver support for workstations
Lack of drivers for some modern personal devices.
Con Not for beginners
If a user needs an easy way to navigate around the filesystem, a text only Unix-derivative is not his best choice.
Con Very shallow desktop support
