When comparing PuTTY vs rxvt-unicode, the Slant community recommends rxvt-unicode for most people. In the question“What is the best terminal emulator? ” rxvt-unicode is ranked 4th while PuTTY is ranked 7th. The most important reason people chose rxvt-unicode is:
Rxvt-unicode uses very little memory and takes a lightweight approach without losing many important functionalities. A single instance of urxvt takes about 6.5MB-8MB of RAM.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Free
Pro De facto standard client for SSH, Telnet and Rlogin on Windows
PuTTY is one of the oldest and most popular clients. It has earned the trust of a great number of users over a long period by being reliable, offering useful features and helpful support. It got into the 15 Essential Open Source Tools for Windows Admins list by InfoWorld.
Pro Can be used on any Windows computer, even without admin rights
Pro Source code available
Full source available. Compile and modify it yourself.
Pro Lightweight and portable
Doesn't require much resources (memory and hard-disk). Can even be run on a system by just downloading without install.
Pro Low memory usage
Rxvt-unicode uses very little memory and takes a lightweight approach without losing many important functionalities. A single instance of urxvt takes about 6.5MB-8MB of RAM.
Pro Can be daemonized to reduce resource usage
For those who want to lower their system's resource usage, rxvt-unicode allows for daemonization. This way you can run several instances of urxvt inside a single process.
Pro Supports terminal transparency
Changing the transparency of the terminal is allowed for in rxvt-unicode. This not only makes for eye candy but is also good for GUI usability.
Pro Minimum dependencies and simple text config
Pro Supports multiple font types
Multiple font types can be displayed flawlessly in rxvt-unicode.
Pro Handles bold text colour properly
Displays characters with text attribute bold actually as bold glyphs.
Pro High performance
Rxvt-unicode is much faster than most alternatives.
Pro Unicode support
International language support is provided through Unicode.
Pro Proper wrapping support
During selection and pasting wrapped text doesn't break into lines at place of wrapping.
Pro Fast text rendering
Rxvt-unicode has very fast text rendering, being able to render hundreds of MB of text in a very short time.
Pro Built-in Perl interpreter
It has a built-in Perl interpreter, meaning that no install is needed. Simply run urxvtperl
.
Cons
Con Only one session per window
Cannot open a second session in the same window (you need multiple windows).
Con Can not save passwords
Anyway, you can use "PuTTYgen" to generate a key pair, then use "Pageant" to do a password-less SSH remote login.
Con Lacks features
It is JUST an SSH client. There are many other options with built in X-servers, Multitabbing, etc.
Con Ugly design, too simple
Looks very dated. Does the basic functions very well, but not much more.
Con No global settings
If you want to change a setting for all your connections, you'd have to do it individually.
Con Organizing sessions
Does not have features for organizing session connections (like folders). If you have 5-10 connections that's fine. But if there are 30-50 connections - that becomes a nightmare.
Con Tedious logging/tracing
Its tedious to set up logging and tracing (e.g. for serial connections).
Con No login scripting
Con Unicode characters that are too wide are blanked instead of clipped
If a character in a backup font is wider than the base font, urxvt substitutes the non-displayable character. A large letter space will show the wide characters, but the result is not reasonably spaced. There is a patch from 2014 that the urxvt maintainer will not merge.
Con Requires work to look acceptable
Out of the box, rxvt-unicode is not too pleasant to look at. Some understanding of Xresources is required for updating its appearance. That being said, the experience of customizing it can be very rewarding.

Con Requires work to configure useful functionality (clickable URL's, font scaling, tabs)
Con In daemon mode you can lose all your terminals
Because of sharing the same process terminal windows cannot be killed without sacrificing all other open windows. That becomes even more dangerous if you use multiple graphical managers without overriding socket with RXVT_SOCKET.
Con Unicode rendering is buggy
There are a few references to urxvt's buggy rendering, such as unicode combining characters, which bled through during scrolling.
Con Bloated
Exists because Xterm is complete trash. Although it has a lot of features, you will most likely never use them all.
Con No support yet for 24-bit "True Color"
Though this is still not available, you can download the patched version for 24bit, here.
Con Font size cannot be changed on the fly with the mousewheel
Sometimes you may need to enlarge the font to improve readability, or shrink it to have more real estate and "hawk's eye" view of data. In rxvt you need to change the configuration file or use these commands:
$ function fontsize
{printf '\33]50;%s%d\007' "xft:Dejavu Sans Mono:size=$1::antialias=false"}
$ fontsize 22
Con No option to have a "reverse color" cursor
While you can choose the color of the cursor and the character under the cursor, there's no automatic "reverse color" for the cursor. This makes it impossible to use rxvt-unicode if you have a light background terminal and a dark background editor.
