When comparing Slackware vs Ubuntu MATE, the Slant community recommends Slackware for most people. In the question“What are the best Linux distributions for desktops?” Slackware is ranked 5th while Ubuntu MATE is ranked 73rd. The most important reason people chose Slackware is:
Uses stable, plain-vanilla packages from upstream.
Specs
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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 Simplicity of Gnome with the power of Ubuntu
Debian was good to use but has limited documentation and is difficult to pick up. Ubuntu MATE brings the ease of design and logic of the Gnome 2 style desktop so it's easy to get around coupled with the vast documentation, forums and ease of learning/using the Ubuntu system.
Pro Works great on old laptops
It does not have a lot of system requirements so it works pretty well even on older laptops.
Pro Perfect for people who like Gnome 2
The goal of MATE is to maintain the look and feel of Gnome 2, while maintaining compatibility with Gnome 3. To that end, it has also forked and renamed many of Gnome's core applications. It benefits from the years of work and polish that have gone into the Gnome project.
Pro Highly configurable
Comes with an option to configure as Windows-like, Mac-like, Gnome-like, etc. in a single click. Wonderful to get you started without having to learn it all from scratch.
Pro Behaves like Ubuntu with Unity - after Ubuntu-Gnome transformation
No better place to continue Unity experience with Ubuntu (Left-side panel, HUD) and not as awkward as Gnome.
Pro Based on Ubuntu
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 No very good Software Center by default
The Software Boutique is not good. You can install Gnome Software from it.
Con Too much bloat included by default
Con Redundant
There is absolutely no reason to install it, since it uses the same package repositories as Ubuntu and you can transform any Ubuntu install into a Ubuntu MATE install.
Con Not bleeding edge
Users who want the latest and greatest software would be happier using Arch Linux, Antergos or another rolling release distro. However, because Ubuntu MATE is based on Ubuntu, users still have access to a large repository of recent software.
