When comparing XMonad vs Gnome, the Slant community recommends Gnome for most people. In the question“What are the best desktop environments for Arch Linux?” Gnome is ranked 17th while XMonad is ranked 19th. The most important reason people chose Gnome is:
GNOME 3 has been designed to make it simple and easy to use. Press a button to view your open windows, launch applications or check if you have new messages.
Specs
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Pros
Pro Fully extensible with Haskell
XMonad is written, configured, and fully extensible in Haskell. This means that users aren't limited to a small set of pre-programmed layouts and actions: anything can be programmed into the configuration.
It's simple to modify basic settings, and the example config has lots of comments to get you started. Haskell keeps this code clean, concise, and readable, and its type system keeps you safe from any serious mistakes. This makes it fast and light, even on very small and slow systems.
Pro Highly configurable
If you enjoy programming, you can even add features to XMonad to make it your perfect desktop environment, and the Contrib modules give you most of what you need to do exactly that.
Pro Efficient to use
XMonad is a very minimal and efficient window manager, especially if the user is familiar with Haskell.
Pro Very stable
Use of Haskell, in conjunction with smart programming practices, guarantees a crash-free experience.
Pro Edit configuration and reload on-the-fly
Configuration is compiled into the WM, and it can be changed/updated on-the-fly, without requiring a full reload.
Pro Great availability of libraries
The use of Haskell as an extension language means that popular pieces of functionality are easily shared and widely available as Haskell Libraries. Many default layouts, and tools for quickly and easily building your own, are available through XMonad-contrib, and highly re-usable configurations are commonly shared through blog articles and the Xmonad Wiki. The documentation in XMonad-contrib is very clear and easy to read.
Pro Xinerama support
XMonad has full support for Xinerama: windows can be tiled and managed across multiple physical screens.
Pro Dynamic Tiling
XMonad uses dynamic tiling which means that it automatically handles arranging your windows into various layouts which the user can cycle through.
Pro Handles multiple monitors well
XMonad can handle multi-monitor setups by default.
Pro Absolutely minimal
The entire window manager is extremely small, and includes nothing beyond basic window manipulation and tiling. Out of the box, there are no window decorations, status bar nor icon dock; just clean lines and efficiency.
Pro Intuitive model which separates "screens" and "workspaces"
XMonad separates screens and workspaces. A screen "projects" a workspace. You can put a window to a specific screen, regardless of which workspace is currently projected onto that screen. This is more intuitive than other WMs e.g. i3, which only has the notion of workspace but not "screen" and requires you to remember workspace numbering. It is especially beneficial for multi-monitor setups.
Pro Extensive documentation
It is sadly a very rare thing among tilling window managers to have comprehensive documentation, Xmonad is the exception to this rule as opposed to those like DWM, and BSPWM.
Pro Friendly community
Especially in comparison to DWM.
Pro Easy to use
GNOME 3 has been designed to make it simple and easy to use. Press a button to view your open windows, launch applications or check if you have new messages.
Pro Very productive
With a clean layout and well-thought keyboard shortcuts, Gnome 3 is simply the best for people looking to be productive with their computer.
Pro Online account management
GNOME 3 integrates with your online accounts, so that all your data can be accessed from the same place.
Pro Adheres to standards
Allowing for interoperability and shared technology for X Window System desktops.
Pro Does not get in the way
GNOME 3 lets you do the things you want without getting in the way. It won't bother you or badger you with demands, and it has been designed to help you comfortably deal with notifications.
Pro Great for high dpi displays
Adjustable scaling factor makes it great for high resolution laptops and far away TVs.
Pro Powerful search
A powerful search feature lets you access all your work from one place.
Pro Easy theming
Changing the look (and feel) of Gnome Shell is easy, shell theme, icon, windows and graphical elements (gtk). individually for each user. Mostly its installing some packages or unpacking some archive to a themes folder and using selecting the new theme in e.g. gnome-tweak-tool. There a lot of really good theme on deviantart.
Pro Fallback mode offers a classic look
For people looking for older, more classic looks, Fallback mode offers just that. Ubuntu users can have this option by installing a package called "GNOME-Session-Fallback." In the future to be released GNOME 3.8, the Fallback Mode will not be included, so this is really not a long-term solution.
Pro Customizable with easy-to-install extensions
With the right plugin enabled in your browser (comes with Firefox by default) you can browse and install with two clicks the many Shell Extensions available from http://extensions.gnome.org. These are listed automatically based upon the version of Gnome Shell you currently have installed.
Pro Touchscreen friendly
It works well with any touchscreen-enabled system, including newer laptops, even to the point of including a well-designed on-screen keyboard.
Pro Keyboard friendly
It's (mostly) usable without touching a mouse, so you can keep your hands on the keyboard. Shortcuts can be defined in the gnome setting. There are even more shortcuts available when using the gesetting or dconf tool, e.g. switch to desktop 5 to 9.
Cons
Con Steep learning curve for uninitiated users
Like a lot of tiling window managers, the learning curve for XMonad is quite steep.
Con Requires knowledge of Haskell for configuration
Understanding of Haskell is required in order to configure XMonad.
Con Requires a lot of Haskell dependencies
XMonad depends on GHC (the Glasgow Haskell Compiler) which can take up about 700 MB or disk space.
Con Doesn't play nice with bars other than Xmobar
Though Xmobar is functional enough, it would have been nice to have a full compatibility to polybar, or at least for it to have more options for improving appearance.
Con Fragile haskell toolchain
Because of its very tight dependency on GHC, which doesn't have a stable ABI, trying to use distro-packaged toolchains can be a nightmare.
Con Has no really powerful layout like bspwm does
Although they have one with the same name.
Con Only tiling
No support for stacking.
Con No taskbar by default
GNOME doesn't have a taskbar by default, and might be confusing to people migrating from Windows for that reason. It is however possible to install it through http://extensions.gnome.org.
Con Some areas are unpolished and lack features
Some aspects are still unpolished and lack features - the gnome design team works well, but seems to not be taking input from the outside world.
Con Extension system is basically unsupported
Backward compatibility is not guaranteed and extensions seems like second class citizens in the GNOME environment.
Con It tries to reinvent the wheel
It isn't a traditional interface. Very different to adjust to.