Both Kali Linux and Ubuntu have expertly crafted Linux distros, with a very distinct use cases in mind.
While Kali Linux focuses more on security testing, Ubuntu favors a set of features that are geared more toward a regular PC user, not an ethical hacker. Both have their place in the Linux ecosystem.
However, Ubuntu can also be for penetrating testing instead of Kali Linux. A good way to achieve that is by installing the PenTesters Framework (PTF) from TrustedSec. The benefit of installing it is that it compiles and builds hundreds of penetration tools on various Linux distributions across, Fedora or Ubuntu, and ArchLinux.
As a beginner, what you should know is that Kali Linux is a bit more advanced for someone new to Linux or even computers in general. Ubuntu, on the other hand, offers a highly intuitive GUI that is easy to understand for anyone. Combine this with the fact that it is completely free.
Kali Linux vs Ubuntu:
It is developed by Offensive Security.
It is developed by Canonical.
The architectural core of Kali Linux is the Linux Kernel.
Its architecture is based on the custom Ubuntu Architecture.
It runs in a GNOME environment.
It uses the Unity environment.
The main purpose of Kali Linux is security and penetration testing.
It is more geared towards everyday home/work use and in servers.
It provides basic mobile support.
The mobile support from Ubuntu is much more robust. It can even be used as a complete replacement for Android.
It isn’t very user-friendly and focuses on functionality over aesthetics and user interface.
Ubuntu is designed from the ground up to be extremely user and beginner-friendly and has an excellent GUI.
It has no bloatware, making it perform super well.
It is also extremely fast and snappy compared to other operating systems but lacks a little behind Kali Linux.
The target users for Kali Linux are digital security experts and ethical hackers.
Ubuntu has no target audience, as it is designed for anyone and everyone.