When comparing XChat vs IRSSI, the Slant community recommends IRSSI for most people. In the question“What are the best IRC clients for Windows?” IRSSI is ranked 8th while XChat is ranked 10th. The most important reason people chose IRSSI is:
Screen multiplexing and remote connection options with screen or tmux.
Specs
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Pros
Pro Highly customizable look
While most clients allow changing the theme or style of a window, XChat gives complete control over what it looks like. It's possible to set a custom image as the background, changing font, or syntax colors.
Pro Works for any level of user
XChat is an advanced IRC client that is good for beginners as well as those users who like to get under the hood. It is easy to get up and running, but has all the advanced features that a power user would need.
Pro Excellent Plugin Library
XChat has a great list of plugins and scripts. Most quality IRC clients support plugins but XChats library is incredibly extensive. There is a wide range of both useful and fun plugins. It's possible to add an RSS feed or play a game of chess. Or maybe you want to control an mp3 player via chat while translating everything you type to l33t speek. XChats Plugin Library is truly extensive.
Pro Great interface
XChat's default interface is clear, well-organized and intuitive. Channels can be laid out as both tabs and in a tree structure. All components such as user lists, topics bar or even menu bar can be hidden to provide a clean way to interact. Conversely the interface can be configured to show quick access buttons for changing channel flags, sending files or performing operator actions.
Pro Terminal-based
Screen multiplexing and remote connection options with screen or tmux.
Pro Themes
Allows the modification of IRSSI's look and feel.
Pro Free and open-source
Pro Quick and easy setup
3 or 4 commands and you're ready to chat with all your old friends.
Pro Perl scripting
Allows the user to create and use perl scripts to perform actions.
Pro Command line outputs can be sent to a channel or message
/exec -o figlet "Slant.co"
Pro Easy logging
Pro Persistent connection
Pro Has a plugin for encrypted communications
Cons
Con Development stopped
The latest release of XChat was in 2010. An active fork is HexChat.
Con No GUI
Actually a plus for some, lower memory overhead.
Con Gives little direction upfront
Actually, Irssi.org has it's own GREAT documentation for new irssi users. Inline /help options are pretty well documented.