When comparing Notepad++ vs Kdiff3, the Slant community recommends Kdiff3 for most people. In the question“What is the best tool for resolving conflicts when merging file changes?” Kdiff3 is ranked 2nd while Notepad++ is ranked 8th. The most important reason people chose Kdiff3 is:
For modern version control systems, 3way merge support is a basic requirement, but many other open source diff viewers do not adequately handle 3way merges.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Syntax highlighting for a wide variety of languages
Notepad++ has built-in support for syntax highlighting for a wide selection of programming languages.
Pro Light and fast
Notepad++ is a very light program that starts almost instantly. This makes it a great text editor for users that want something that will start the second they open it.
Pro Extendable via plugins
A list of hundreds of plugins is maintained.
Pro Portable
You can get a portable version of N++ and put it on a flash drive or your dropbox account and have your editor, configured the way you like, at any computer that you are on.
Pro Free under GPL
Notepad++ is licensed under GPL, which means it is free/open source software that you can use freely.
Pro Regex replace in selection, active tab, or all tabs
In Notepad++, the user can utilise regular expressions to quickly modify text across multiple files.
Pro Persistent documents, even after exiting the application
If you close Notepad++ (npp), your documents remain even if you haven't saved.
Pro User defined language syntax support
You can define your own custom syntax highlighting rules (or add support for others) .
Pro Easy to use and admin
Very easy to use and personalize.

Pro Multi-line editing
While it is disabled by default, when enabled, it is possible to edit more than one line at a time. This is helpful in many situations.
Pro Split screen
The user can open and edit files in multiple screens within the editor window.
Pro Supports Markdown
If you have the Plugin Manager installed you can search for MarkdownViewer++ and install it via that plugin.
Pro Supports 3 way merges
For modern version control systems, 3way merge support is a basic requirement, but many other open source diff viewers do not adequately handle 3way merges.
Pro Free and open source
KDiff3 is completely free to download and use. It's also open source released under the GPL.
Pro Can compare directories
It is able to compare whole directory trees.
Pro Supports editing files directly
In addition to comparing two files it also allows you to edit the merge result right in place.
Pro Diff by character not by lines
On comparing two files, difference is shown by characters; not by lines.
Pro The UI is customizable
Allow customizing colors regardless of user/system theme.
Pro Supports manual code aligning
With selecting code in one window and hitting Ctrl+Y, then selecting some other code in second window and also hitting Ctrl+Y you can manually align the code.
Pro Preprocessing before calculating differences
There are options that may pre-process compared files before Kdiff3 actually do a comparison - to ignore for example automatically generated dates and/or revision numbers added by commit hooks.
Pro Context menu shortcut
You can right-click a folder/file and the options:
- Save <file> for later
- Compare with
will be available, making launching KDiff3 really convenient.
Cons
Con Windows-only
While it can run in Wine, it is native only to Windows. Linux users will have to use Notepadqq instead.
Con Settings confusingly scattered
Examples: try to change the tab size or used font.
Con Annoying update notifications upon start-up
Annoying update notifications tend to pop up upon start-up after not having used the app or machine for a few days. At the same time, they can be easily turned off.
Con Outdated UI
Only the text area can be themed, and it doesn't have as many features as browser-based text areas.
Con Limited new syntax support for new languages
It may be hard to find good plugins for relatively new languages.
Con Session backups not enabled by default
Unsaved tabs will be lost when Npp crashes, unless you first enable the session backup option.
Con User defined language doesn't support triple quote strings
It also doesn't support triple hashed comments. Both styles are overridden by their single character single line version.
Con Cannot do inline diffs
Comparison of 2 files is always side-by-side and there's no option for inline views. Overall a rather poor and confusing UI in general.
Con Confusing GUI
4 sub-windows (when you really only need 3), a lot of different colors and even more confusing result-window. No links what has changed between versions and and the result. It clearly shows it's dated or rather outdated. Great if you ever need to do a command line merge, otherwise it sucks.
Con No precise editing of the compared files
Precise work line-after-line is not possible. Only a version after the automated merge-step is editable, but not the two files separately.
Con No longer supported by Homebrew for MacOS
Cannot be installed easily on Mac as of Aug 2019.
Con Problems when files have different number of lines
For example, if you add 3 lines: A, B and C locally but on the other change there are only A and C, Kdiff3 will work out that A was added, then it says that B conflicts with C but adds C again anyway.
Con Slow for large files
Con No image compare
Compare is text based.
