For the last four years, Debian and Ubuntu have been in the top three Linux distributions on Distrowatch. Since 2005, neither has been out of the top six. But which Linux distro is right for you? You can't go seriously wrong either way, but a useful answer depends upon what you want in a distribution.
You may have heard that Debian is a distribution for experts, and Ubuntu for beginners. That is true, so far as it goes. However, that distinction is more historic than contemporary.
It is true that after Ubuntu burst on to the scene in late 2004, it spent several years making the desktop easier to use, especially for non-English speakers. However, thanks to free licenses, Ubuntu's improvements have spread to most desktop environments.
Moreover, Ubuntu's days of interface innovations are largely in the past. Today, Ubuntu development is focused largely on convergence -- the development of its Unity desktop into a common interface for phones, tablets, and desktops. But since Ubuntu phones and tablets have limited availability, convergence is largely irrelevant to many users. Similarly, Canonical, Ubuntu's parent company, seems more focused on its successful OpenStack division than on desktop development.
By contrast, modern Debian is easier to install than ever before, and is imitating Ubuntu with Long Time Support, starting with the present release. Modern Debian is also focused on its role as the source of other distributions, including not only Ubuntu, but also Linux Mint, the distribution that has been consistently first on Distrowatch in the last four years.
Today, Debian can best be described as a distribution for everyone, easy to install for anyone who can follow instructions, but with the potential for detailed control that will satisfy experts.
Yet these general descriptions are only part of the differences. Despite their common origin, Debian and Ubuntu differ in their installers, desktop environments, administration, and communities as well.
Contrary to the lingering myth, modern Debian is no harder to install than any other distribution. In fact, with the Debian Installer available for eleven hardware architectures and in text, graphical and voice formats, and with an expert mode for troubleshooting, if you cannot install Debian on your hardware, then very likely you will be unable to install any Linux distribution at all.
Ubuntu's installer, though, concentrates on 32 and 64 bit versions of Intel architecture for desktops and servers, although in recent years it has moved into ARM development as well. Nine alternatives or "flavors," are available, ranging from ones for specific purposes, such as the educational-oriented Edubuntu, and the Chinese language Kylin to ones with different default desktop environments, such as Lubuntu, which installs LXDE and Xubuntu, which installs Xfce.
By default, all these choices for Ubuntu use an installer designed to require a minimum of user input. However, in the event of difficulties, Ubuntu uses an expert installer that is a rebranded version of Debian's.
Originally, Ubuntu used GNOME as a default desktop. But since 2010, it has developed its own desktop called Unity. Yet although Unity is intended to be easy to use, it has never been particularly popular. Fortunately, a wide variety of alternative desktop environments are available after installation, although Kubuntu, the KDE Ubuntu, is now developed independently, and no longer has a link on the Ubuntu web site.
Debian, too, defaulted to GNOME for many years. The most recent installer, though, offers half a dozen desktop environments, saving time for those who like to have multiple desktops. The difference lies mainly in when and how you install multiple desktops.
Debian installs with a root account and at least one non-privileged user. Unlike Ubuntu, it does not use sudo, which hides the root password, and allows at least one user access to root privileges by entering their own password. Using sudo has the advantage of minimizing the time that the root account is in use, but whether Ubuntu's sudo set up is more secure than Debian's classical arrangement is much debated. With both Debian and Ubuntu, users might choose another sudo setup that is more in agreement with their concepts of security.
Debian's packages pass through the Unstable and Testing repositories, ending in Stable with the release of a new version. In between releases, Update and Backport help keep Stable current, and users can also choose between the self-explanatory Old Stable and Experimental. All these repositories are divided into three sections: main, which contains free- licensed packages, contrib, which contains free-licensed packages dependent on proprietary software, and non-free, which contains proprietary packages. Of these three, only main is included on the install DVDs; contrib and non-free must be added to the sources by the user.
This arrangement makes Debian technically free -- if not recognized as such by the Free Software Foundation -- while providing users with other packages they might want and encouraging the non-standard packages to maintain a higher standard than they would probably have if, like Fedora, all non-free and contrib packages were maintained by third parties.
In comparison, Ubuntu repositories are organized by the organization responsible for them. Main contains software supported by Canonical, while software in Universe is supported by the Ubuntu community. Proprietary drivers are in Restricted, and software with legal complications is placed in Multiverse.
Both distributions use the .deb format, and many packages will run in either distribution. Usually, third party packages will also run in both so that only one package needs to be maintained, but, increasingly, some packages built by Ubuntu will not run in Debian.
In particular, Ubuntu is developing Mir as a replacement for the X Window System that provides Linux's graphical display, while Debian, like most distributions, is moving towards using Wayland as a replacement.
In addition, starting in 2016, Ubuntu has also promoted Snap packages. So far, there is no official plan to replace .deb packages with Snap ones except in hardware, but many speculate -- contrary to official statements from Ubuntu -- that such a change is only a matter of time.
Unlike .deb packages, which depend on other packages for dependencies, a snap package includes its dependencies in the package, making them a sort of updated static tarball, and runs each package in a container. Ubuntu claims that snap packages are easier to make, and more secure, but it might be argued that the quality of a package depends, not upon the technology, but the testing process.
Also, with the duplicates of dependencies in different packages, snap packages also occupy more hard drive space.
Debian is a community distribution, in which all those who have passed the tests to become a Debian maintainer and selective non-maintainers elect the project leader and vote in referendum on major policy issues. Many decisions, especially on legal matters, are debated at length in Debian forums, especially debian-devel and debian-legal.
Ubuntu, though, is more complicated. On day to day matters, Ubuntu runs like a community distribution, with a Community Council and various sub-committees. However, Mark Shuttleworth, Ubuntu's founder, has appointed himself Benevolent Dictator for Life, and has been known to over-rule community decisions in the interests of his company Canonical Software. Once or twice, this arrangement has led to near community revolts, and complaints about the lack of a clear definition of community.
Despite these differences, the interaction between Debian and Ubuntu remain close. Although some Debian members feel that Ubuntu does not give Debian enough credit, others have been known to work for both distributions. Members of both distributions also attend the annual Debconf conference.
Yet for users -- especially potential contributors -- the two distributions offer increasingly different choices. Debian, for instances, remains a general purpose distribution, while Ubuntu increasingly reflects the business interests of Canonical. Recent Ubuntu releases have offered little new for desktop users, but the same might be said of Debian, since it has reached a level of maturity where the gaps in software are increasingly few.
If I had to give advice, I would suggest that Ubuntu with some desktop other than Unity would be ideal for newer users, and Debian for more experienced users or for new users who wanted to learn about Linux without switching distributions.
Yet, that said, the technological basis of Debian and Ubuntu remain similar -- and, really, both are among the best choices for the average Linux user.