Most free software users have long ago made their decision about whether they prefer the GNOME or KDE desktop. However, with the release of GNOME 3 this week, the choice requires a new answer. Now, both GNOME and KDE have versions that are radically different from those each had a few years ago.
The difference is not in the software. The choice of applications designed for each desktop has not altered: in most cases, the utilities have identical features, but GNOME still lacks a font installer and a fully-featured music player, while KDE could use better accessibility and network connection tools.
However, if you look at system requirements, desktops and workflows, and general design philosophy, the differences in KDE 4.6 and GNOME 3 (the latest releases) are greater than they have ever been.
I couldn't find any list of system requirements for GNOME 3. The GNOME 3 FAQ does state that "Computers purchased in the last 4 or 5 years should be more than capable of running GNOME 3," but this generality fails in at least two instances.
Running nothing except a virtual terminal, GNOME 3 consumes 883 megabytes of RAM on a one gigabyte notebook. Not only is this requirement more than twice that of GNOME 2.32, but it compares unfavorably with the 615 megabytes needed by KDE 4.6. Realistically, it means that two gigabytes of RAM would be a realistic requirement for GNOME 3 unless you have the patience to continually use the swap partition. That probably means that, unless your machine is less than a couple of years old, GNOME 3 is not the desktop that you should be using.
More importantly, while not having hardware acceleration in KDE only prevents you from running some compositing effects, GNOME 3 requires basic hardware acceleration to run at all. Most Intel video drivers are sufficient, but with some other chip sets, proprietary drivers may be necessary, which free software advocates will find unacceptable. Admittedly, GNOME 3 is supposed to come with a "complete fallback interface," according to the FAQ, but the fallback was unavailable on the Live CDs I used for this comparison.
In earlier versions, GNOME and KDE each offered a distinctly different appearance -- GNOME minimalist and professional, KDE artistic and casual. However, the latest versions, with their translucencies and abstract wallpapers closely resemble both each other and Windows and OS X -- so much so that, in ten years' time, everyone will be able to identify any of these desktops as coming from the same era.
Beyond their appearance, both the latest versions also represent efforts to evolve the basic desktop metaphor that has remained largely unchanged for over twenty years.
For both desktops, this evolutionary effort involves creating an interface that is suitable for everything from a workstation to a mobile device. However, the two solutions could hardly be more different.
One of the architectural changes in the KDE 4 series of releases is the abstraction of everything from hardware and personal information management to sound and interfaces into sub-systems that applications can access instead of managing such things themselves. Under this arrangement, only the interface needs to be changed to adapt KDE for a particular use, and developers have to maintain only minimally different desktops for different use cases.
KDE calls these interfaces "containments." If you open the context menu and select Folder View Activity -> Activity -> Type, you can see some of the pre-defined ones, such as the Newspaper Activity and the Search and Launch Containment, both of which were originally designed for netbooks.
Following the same logic, KDE does not have a single desktop where you can place icons. Instead, you can maintain multiple sets of icons in an object called a Folder View, and quickly change between them according to the current task, either by changing the set on your current desktop, or by switching to a different virtual desktop, or Activity, as KDE calls it.
At first, you might think that GNOME 3 has a similar design, especially since its panel has a link to an Activities screen. However, when you click the Activities link, you immediately realize that the term is being used differently than it is in KDE.
Like KDE's, GNOME 3's Activities are virtual work spaces (or something close to them). However, unlike KDE's, GNOME 3's Activities are not each a fully-equipped desktop. Instead, each is a stripped down interface with a panel that contains no applets or widgets -- just a task bar, clock, controls for sound, Internet, and chat, and logging out, plus the Activities link.