When comparing WeeChat vs Communi, the Slant community recommends WeeChat for most people. In the question“What are the best IRC clients for UNIX-like systems?” WeeChat is ranked 2nd while Communi is ranked 14th. The most important reason people chose WeeChat is:
WeeChat has a command line interface.
Specs
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Pros
Pro Terminal-based
WeeChat has a command line interface.
Pro Accessible to less expert users out of the box
WeeChat has an easy to use interface and sane defaults, making it a great client for beginner users.
Pro Builtin SSL support
Easy to configure weechat connect to server using certificates
Pro Highly customizable and configurable
Many dotfiles are publicly available.
Pro Flexible window splitting
Weechat allows very flexible window splitting behavior to show multiple IRC channels at once. Windows can be split vertically and horizontally.
Pro Fully keyboard driven
Weechat can be fully driven by keyboard shortcuts. Navigating between buffers, switching between windows and changing text styling can all be achieved with keyboard shortcuts.
Pro Extensible via plugins and scripts
Supports scripting in C, Python, Perl, Ruby, Lua, Tcl, Scheme and Javascript.
Pro Persistent connection
Works perfectly with a poor connection.
Pro Smart filtering
Smart filtering lets set a series of rules for IRC channels. For example, it allows instructing the client to show /quit and /join messages only if the user was speaking in the last 20 minutes.
Pro Free and open source
WeeChat is a free program licensed under GPL with source code available on GitHub.
Pro Can act as a relay for mobile client
It's possible to leave WeeChat running at home/on a server, then connect to it with a smartphone client. This way a choppy mobile internet connection won't disconnect from IRC, and still allow using a touch screen optimized UI.
Pro Active and helpful community support
Weechat community can be found on #weechat @ Freenode. The community, including devs, is helpful in answering any WeeChat related questions.
Pro Built-in script manager
WeeChat has a built-in tool for searching, downloading, installing and updating scripts and plugins.

Pro Intuitive, detail-oriented interface
By default WeeChat has a chat area surrounded by a nicklist on the right, title bar on top, and status and input bars at the bottom. The layout, though, can be completely customized and split vertically or horizontally. It supports 256 colors as long as the terminal supports 256 colors as well as bolded, italicized and underlined text. Even includes little touches as differently colored nicknames for people /away.
Pro Huge library of scripts
WeeChat scripts repository has hundreds of scripts including url shorteners, music player integrations and bandwidth monitors.
Pro Mouse support
Mouse click and gestures are supported on a local terminal as well as remote terminal accessed via ssh regardless of screen/tmux. To enable mouse support use /mouse enable
command or press alt+m
.
Pro Easy to customize
You are able to modify configurations inside application or via modifying files(i.e. colors).
Pro Actively developed
Pro Offers a choice of remote interfaces
WeeChat has html5, qt, android, emacs and javascript remote interface options.
Pro Incremental text search
Text is searched through in real time as search terms are being written.

Pro Elegant and minimalist interface
Communi user interface for desktop and mobile is minimal, which helps you focus on the task at hand instead of being distracted by extra UI elements.

Pro Good ZNC integration
The author is involved in ZNC development, so the latest ZNC goodies are guaranteed to be well supported (http://communi.github.io/znc/). With help of the documented ZNC modules, Communi retains its state well across sessions. No repetitive buffer playback on reconnect, notifications and banners for newly received messages only, queries stay open until closed etc.

Pro UI runs on multiple platforms
While Communi itself is a IRC framework, the project offers several UI implementations such as desktop, SailfishOS, Meego and Symbian.

Pro Quassel protocol implementation
Communi is integrated with Quassel.

Pro Event merging
Consequent events (joins, parts, quits etc.) are merged to reduce noise and to keep focus on the discussion.
Cons
Con No official GUI option
While there are some unofficial remote graphical interfaces available for WeeChat, it is intended to be used as a terminal client. As such it might be challenging to use to those accustomed to graphical interfaces.
Con Scripting API allows poorly written scripts to be used as attack vectors
Plugin documentation includes the following warning:
In many functions, for speed reasons, WeeChat does not check if your pointer is correct or not. It’s your job to check you’re giving a valid pointer, otherwise you may see a nice crash report ;)

Con No DCC
Communi does not support DCC (for a reason)

Con No scripting
Communi has no support for scripts
