When comparing Slackware vs Gentoo Linux, the Slant community recommends Gentoo Linux for most people. In the question“What are the best Linux distributions for desktops?” Gentoo Linux is ranked 3rd while Slackware is ranked 5th. The most important reason people chose Gentoo Linux is:
You build the package from a source you can see and read. You decide which features you want to build in and which aren't needed. You can choose build options, optimisation and whatever else fancy stuff you want modified. With a binary distribution this simply isn't possible.
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 Full control of the software
You build the package from a source you can see and read. You decide which features you want to build in and which aren't needed. You can choose build options, optimisation and whatever else fancy stuff you want modified. With a binary distribution this simply isn't possible.
Pro Portage's emerge is powerful
Portage is a package management system with a huge amount of options and features.
Pro Fully customizable
The usage of advanced features like USE flags makes it more customizable than any binary distribution.
Pro Freedom
You are free to do whatever you want with it. As a result, its configuration reflects your unique taste and personality.
Pro Great for anyone who is serious about learning the intricacies of Linux
It's useful for both beginners and professionals. For the installation, Gentoo offers various types, which are referred to as stages. Basically meaning how in depth you would want to go into the process of installation. For beginners it's useful to choose for a starting distro due to its various stages that can be very time consuming but beneficial as you learn the composition in general of Linux.
Pro Can by optimized to any given CPU by using proper compile flags
Since everything is being built on your PC you can fine-tune the code to make use of your CPU. And all it requires is two lines of string variables in a global config.
Pro Portability
There are many architectures available for Gentoo : i386, x86-64, PowerPC, PowerPC 64, sparc, DEC Alpha, ARM, MIPS, PA-RISC, S390, IA-64, sh, m68k.
Pro Can be officially systemd-free
Gentoo's default init system is OpenRC. Gentoo also officially supports systemd-free Gnome and udev. Users are free to choose any init system they want.
Pro Documentation and community are second to none
Provides a whole handbook to refer to during setup and usage.
Pro A very large collection of software is available
There are more than 19.000 packages available in the official repository. And even more with overlays.
Pro Gentoo does not impose a standard look-and-feel
Pro Superiority and smartness
You get superiority and get called smart daily for going through the processes of punishing yourself to a compile install you also get a cool neofetch logo.
Pro Live USB installation
Installation via a Live USB allows you to go back and fix mistakes from it without having to restart the whole installation process.
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 Not beginner-friendly
You have to read a lot of instructions to start, even if you are familiar with Linux. Furthermore, as you have to configure the kernel and init system, expect some boot failures at beginning.
Con Since everything must be compiled it takes a lot longer to update
And if you have to update the kernel or some heavy software like Chromium it can takes hours if not more on weaker hardware.
Con No default installer
Installation must be done manually by following instructions in the handbook.
Con Customized package installation can take a long time and cause installation failures
The Gentoo package management system allows you to configure what compilation flags packages should support - i.e. specific processor flag support (SSE, SSE2, etc.), -O1, -O2, -O3 optimization, etc.
If you accept one of the default flags, Gentoo downloads binaries from the server. However, if you decide to optimise, it can and will download all source packages and start compiling allthe programs and libraries on your system. If your chosen flags don't work with a particular library, installation will fail.
Con Somewhat outdated solutions
While being outdated per se is virtually impossible for a rolling-release distro with a large community, a large portion of said community sticks to outdated solutions. For example, Gentoo's primary init system is OpenRC, which is cumbersome and awkward to use and provides little control over the system. While you can just choose systemd, it will require some tinkering. Other examples include stubbornly declaring an initramfs a last resort and an "oh my god 1337 H4XX0RZ surely have nothing better to do than trying for a month to exploit some vulnerability to steal my pony art, I have to fortify so hard my performance and ease of use will suffer" 90s security mentality.
Because of just how much freedom Gentoo provides you with, this usually isn't a big deal though.