When comparing Neovim vs GVim, the Slant community recommends Neovim for most people. In the question“What are the best open source programming text editors?” Neovim is ranked 3rd while GVim is ranked 15th. The most important reason people chose Neovim is:
The core text editor is "headless", meaning it's detached from the user-interface so other programs can hook into it. This enables better integration with IDEs and browsers, where "Vim mode" has typically been a poor substitute because it was a partial rewrite or a partial port at best. One of the advantages of Vim has always been ubiquity and Neovim makes it even more ubiquitous.
Specs
Question Ranking
Pros
Pro Better integration with external tools
The core text editor is "headless", meaning it's detached from the user-interface so other programs can hook into it. This enables better integration with IDEs and browsers, where "Vim mode" has typically been a poor substitute because it was a partial rewrite or a partial port at best. One of the advantages of Vim has always been ubiquity and Neovim makes it even more ubiquitous.
Pro Still Vim but with upgraded features and some issues fixed
NeoVim was a complete rewrite of Vim, with new features added and underlying issues resolved thanks to the Vim code base. The keybindings and configuration are the same as Vim, so the switch can be pretty simple.
Pro Powerful plugin model
Vim plugins have always been useful, but tied to specific languages. Neovim's architecture provides better separation between plugins and the core product, so that plugins are completely flexible and can be written in any language.
Pro Modern code base
As a refactor over Vim, Neovim has greatly improved its code base. For example, some functionality is handled by libuv, the same code base that powers Node.js.
Pro Built-in terminal emulator
This avoids the user having to make any installations.
Pro UI Agnostic
The core functionality is handled apart from the UI, meaning that Neovim can be embedded into any other GUI system, such as Atom.
Pro Async plugin execution
Pro Active development community
Pro Comes with some good configurations out of the box
Some typical configurations most of VIM users make are default in Neovim.
Pro Opens a 3Gig Text File in a few seconds
Not many editors can open such a large text file so quickly.
Pro Fast and light on memory usage
New neovim editor instance starts instantly and you can have multiple editors open at the same time, because id does not require a lot of memory to run.
Pro Work in TUI (Tex User Interface)
Neovim can work on terminal, on a remote server over ssh.
Pro Terminal mode is very convenient for testing code in a split window
Pro Vim without a shell
When opening files from the file manager, it is still nice to have vim.
Pro Excellent performance
Because it loads the whole file into RAM, replacing all string occurences in 100MB+ files is quick and easy. Every other editor sort of died during that.
Pro Works on all platforms
Whether its your windows machine, a Linux, Unix or a Mac Vim would work everywhere. You can even build it from its source on your favorite linux environment.
Pro Has different cursor shape in command and insert modes
Unlike vim, gvim has different cursor shape in command and insert mode, which is very convenient.
Pro Supports all vim features and settings
All vim features, custom settings, and plugins are automatically available.
Cons
Con Ambiguity in extensive documentation
Con No graphical editor yet
At the time of writing this, no equivalents to gVim exist.
Con Limited cross platform support
Neovim is not available for many legacy platforms
Con No stable release yet
As of yet, Neovim is in an unstable point in it's development. There is no stable release and using it for the moment should be done with caution as many features may change in the future.
Con Poor feature discoverability
Con Poor support for external tooling
Con High effort to customize
A lot of time and effort is put in to make it specific to your needs.
Con Requires Brain Mode Switching
When editing in vim, you have you use the vim keys; when editing in every other window on your PC, or in Word or Excel or other application, you need to use the standard system key combinations. Learning the vim combinations can actually make you SLOWER at everything else.
Con Consume brain energy for editing that should be used for logic
Text editing in vim can be great once you've learned it, but it requires thinking about combination of commands. In other editors, you don't have to think about how to delete this part of code. You just think about how to implement a feature, what is a good design for this code. Even after you get used to using vim, it still requires your brain for editing.
Con Modal editing
Has a split between editing text (Insert Mode) and the other functions of the editor. This can distract you from thinking about what you're trying to accomplish.
Con Split the VIM community
Moolenaar to be blamed for. If he opened up the development for vim to other bright minds, no fork would have happened. As it is mostly compatible with vim, it is not such a big issue.
Con GVim, like Vim, has a difficult learning curve
A lot of time is needed to learn all the commands and modes supported. A lot of time will also be spent tuning all the settings to the user's preference.