When comparing Conky vs Glances, the Slant community recommends Glances for most people. In the question“What are the best system monitors for UNIX-like systems?” Glances is ranked 2nd while Conky is ranked 3rd. The most important reason people chose Glances is:
Unlike the default system monitors found in Ubuntu or Gnome Shell, Glances shows a lot of information all in one place instead of having to click through tab after tab. This makes cursory glances at one's stats a lot easier and less time-consuming.
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Pros
Pro Infinitely themeable
Conky is editable to the point that there is almost no limit to the designs one can use to display the type of information one wants. Not only are there thousands of themes available on sites like deviantart and gnome-look, but a user can create any type of look they like with a little bit of know-how and imagination.
Pro Many different sensor stats
Conky has a bunch of sensor stats that the user can choose to display such as CPU usage, memory, uptime, etc. Depending on what the user wants to display, most likely there is a way to display the needed info when using Conky.
Pro Runs on top of X
Pro Important information all in one place
Unlike the default system monitors found in Ubuntu or Gnome Shell, Glances shows a lot of information all in one place instead of having to click through tab after tab. This makes cursory glances at one's stats a lot easier and less time-consuming.
Pro Includes network bandwidth
Pro Works in a terminal
Works perfectly well even if you remote access a server via SSH.
Pro A lot of information
Much more information than htop.
Pro Easily installed
Glances is a popular system monitor and in being so is often found in most distros repositories, so it is easily installed.
Pro Web interface and remote monitoring features
A Web interface based on a Restful JSON API is also available and allows remote monitoring.
Pro Shows only necessary details
Most performance monitors overwhelm you with too much detail. Glances almost always shows exactly what you were interested in immediately.
Cons
Con Setup can be difficult
Being text based, the config file for Conky can be intimidating for those unfamiliar. Though there are configuration managers out there such as Conky Manager which can make the setup a bit easier.
Con Is system monitor but can't manage processes
Conky is a great system monitor, but can't e.g. kill (individual) processes like htop can.
Con Requires X
Cannot easily be used on a headless server or remotely (most people and servers support remoting via SSH).
Con Lack of colors can make it confusing.
Con High cpu usage (on RPi), broken after a few hours of running
It's based on Python.