When comparing Neovim vs IDLE, the Slant community recommends Neovim for most people. In the question“What are the best Python IDEs or editors?” Neovim is ranked 9th while IDLE is ranked 24th. 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 Active development community
Pro Async plugin execution
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 Terminal mode is very convenient for testing code in a split window.
Pro Sometimes simple is best
For short scripts, a heavyweight IDE just gets in the way. It's also easier for beginners to understand.
Pro Written in pure Python/tkinter
You can dig in and change how it works.
Pro Included in standard Python distributions
You probably already have it.
Pro Debugger
It has one.
Cons
Con Limited cross platform support
Neovim is not available for many legacy platforms.
The Windows version is currently considered experimental.
Con Ambiguity in extensive documentation
Con No graphical editor yet
At the time of writing this, no equivalents to gVim exist.
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 Why clone Vim
You can use Vim and plugins.
Con No Tabbing for Files or Shell instances
Idle's Interactive Python Shell and the Python Text Editor are separate window applications. Many would expect them to be unified together within a single window. To create a python program file, or module, the user first opens Idle (It's Interactive Shell), then the user goes to [File] and [New File] to open the Idle Text Editor. Plus, every time a new python file is opened, a new instance of Idle runs separately. So, there are no tabbed modules. That's clumsy approach that adds the complexity of juggling around many Idle instances.