When comparing Pidgin vs Communi, the Slant community recommends Pidgin for most people. In the question“What are the best IRC clients for UNIX-like systems?” Pidgin is ranked 6th while Communi is ranked 14th. The most important reason people chose Pidgin is:
Pidgin not only supports IRC, it also supports plenty of other services such as AIM, Google Talk, MSN, ICQ and plenty of others. This way, you can have all of your conversations in the same app and won't have to deal with multiple chat windows being open.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Support for a wide range of additional chat services
Pidgin not only supports IRC, it also supports plenty of other services such as AIM, Google Talk, MSN, ICQ and plenty of others. This way, you can have all of your conversations in the same app and won't have to deal with multiple chat windows being open.
Pro Free and open source
Pidgin and its code is completely free and open source. Pidgin has no ads and no features behind a paywall.
Pro Cross platform
The Pidgin instant messaging client is cross platform, working on Linux, Windows and OSX.
Pro Convenient notification features
Like other quality IRC clients, Pidgin features notifications that the user can set in order to have a variety of things happen when the users is mentioned, such as an audio cue or changing the system tray icon.
Pro Highly customisable with plugins
Pidgin not only has a large plugin library built-in, but also an array of third party plugins for a variety of functions.

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 Miserable IRC font controls (as of 2.13.0.17)
Does its best to avoid putting sensible default font controls in any place where you can get to it. And when it does grudgingly change it, based on system preferences (not local preferences), it does so in an inconsistent and unpredictable fashion.
Con No native VoIP support
While there are plugins there is no built in support for VoIP in Pidgin which is time consuming to implement instead of being baked in from the jump.

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

Con No scripting
Communi has no support for scripts
