Habit is the enemy of change. If you have been using Linux for a while, you may have gotten used to the distribution it offers. If your situation and computing needs changing, then you should think it over. If not, you might want to consider learning a new system for the benefit of apprehension. Knowledge is a very light burden to bear.
The Gentoo system is aimed at savvier users, as such, it is inconvenient to start with. As an example, your installer is a command-line only, which means you need to compile the software, so you installed. However, there are some exceptions to this.
When you choose Gentoo, you must be prepared for the command-line. Other than that, you also need no default desktop. If you want a little bit more convenience, use a derivative. You can find a comparison of derivatives in this article.
While using a command-line installer seems to be inconvenient, it is advantageous once you get used to it. The Gentoo command line package manager, in particular, has a surprising number of features, including news! With this, you can read the latest news about Gentoo from the command line.
If you have a problem finding your application, you can add support for Flatpak with a package. If you want to use AppImage, you will need the required libfuse, which Gentoo delivers as sys-fs/fuse.
Ubuntu is a polished version of Debian. Packages included will help you in finding derivatives easier. This is the most popular distribution delivered by Canonical. You will always find references to Ubuntu when dealing with Linux.
You are presented with a great graphical installer when you choose Ubuntu. Before you start, remember to choose your favorite desktop environment. The default is GNOME, if you want KDE, then you need to choose Kubuntu. If you have other preferences, there are a variety of pre-sets available. If you start with the wrong one, you will end up with many unnecessary packages.
Adding software can be done in many ways. Standard is of course their own repositories, which uses Debian format files. Secondly, they have also chosen and designed snap as the secondary default. Aside from that, you can also use Flatpak and AppImage.
Basic Differences in philosophy
Ubuntu uses the Debian package manager, aimed mainly at binary packages. Though source code is available for most of the packages, this makes the installation faster. Also, hopping between desktop managers is less risky. Gentoo aims to deliver the source code and let the installer compile it for the platform you are using or plan to use. Gentoo tries, and usually succeeds, to create a system that is extremely optimized for the particular system you use. You can actually pick the specific CPU model if you wish. With the USE variable, you can force the binary to only support your specific desktop. Apart from that, you can compile every package and install it on both systems. Nevertheless, the philosophy leads to very different default behavior. This led to many flame wars.
The packages in Gentoo contain links to the upstream and seldom has any source code included. The bulk of the package system helps you set the compilation options and handle patches.
Packages in Ubuntu, in contrast, contain the entire binary or source code. Dependencies are controlled by both systems, though in Gentoo, you can use parameters called slots to have several versions installed. In Ubuntu, you must go through hoops to get several versions, though, with applications, you can use an AppImage.
Usage and Install Differences
The main objective of Gentoo is to optimize the system on every installation. As mentioned before, this leads to a long and, often, slow installation procedure. Proponents claim that this will lead to a faster and more stable system. This is probably true as long as you take the time to do the job at the install. For small systems, it is a good idea to run your installation with a distributed compiler, like distcc. This would speed up the compilation while leading to a more optimized system. For many users, this is a nuisance. Nonetheless, for both cases, you have special distributions and procedures for the popular Raspberry Pi. This includes an optimized Stage3 file, which is a basic system that can run on the target systems. The procedure is here, it shows you how to download the image and the Stage3. On top of that, you get a short introduction to cross-compiling using the Gentoo package ‘sys-devel/crossdev’. That package supports the Armv6, so you can install on the original and, crucially, the Pi Zero W.
For Ubuntu, you can choose and install images from their website. They are ready systems that aim to be servers or desktops of your choice. You can also use the source packages and compile the thing wherein it is less complicated as compared to using Gentoo.
On larger systems, like a laptop, you must measure performance or have exceptional demands to make a significant difference. Many lovers of Gentoo choose a suitable derivative and sticks with that. When they feel that they need an improvement, they recompile relevant parts of the system.
Who is the winner?
This is a very subjective question. The answer may differ for the same user from different circumstances. Ubuntu wins on ease to start, several of packages, and convenience. Gentoo has the advantage of becoming the most performance-wise, just as their goal is. You must choose what your priorities are and go for it. The number one thing that you must consider is the distribution that matches your needs for your system. Many times, Gentoo will win, but only after you have put in the effort for the right reasons.
For many users, choosing Gentoo is a giant leap. A leap that they never take but can be a serious mistake if you have important reasons to use your computer or system of computers.