'xclip' vs. 'xsel'

Both xclip and xsel can store text into 3 different selections (by default it is primary selection). From experience I know that primary selection is basically what you high-light and released with the middle mouse click (which corresponds to pressing both right and left touchpad key on a laptop). The clipboard is the traditional CtrlV.

By examining the man pages for both, however, I've discovered that xclip wins in one aspect - reading from an input file:

xieerqi:
$ cat testfile.txt                                                             
HELLOWORLD

xieerqi:
$ xclip -selection clipboard testfile.txt

xieerqi:
$ HELLOWORLD
mksh: HELLOWORLD: not found

xieerqi:
$ xsel testfile.txt 
Usage: xsel [options]
Manipulate the X sele . . . (usage page goes on)

Of course you could use shell redirection with xsel to get around that

xieerqi:
$ xsel --clipboard < testfile.txt                                              

xieerqi:
$ HELLOWORLD
mksh: HELLOWORLD: not found

xclip also wins in the fact that you can output the contents of clipboard to file (which is perhaps useful when you want to redirect PRIMARY selection , i.e. highlights). xsel offers only output to stdout

In addition to @Serg answer, there is a piece of information from the Tmux page in the Arch Wiki that can be useful in some specific cases:

unlike xsel it [xclip] works better on printing raw bitstream that doesn't fit the current locale. Nevertheless, it is neater to use xsel instead of xclip, because xclip does not close STDOUT after it has read from tmux's buffer. As such, tmux doesn't know that the copy task has completed, and continues to wait for xclip's termination, thereby rendering tmux unresponsive. A workaround is to redirect STDOUT of xclip to /dev/null

Something else to keep in mind, xsel has fewer dependencies than xclip:

# apt-cache depends xsel
xsel
  Depends: libc6
  Depends: libx11-6
  Conflicts: xsel:i386

# apt-cache depends xclip
xclip
  Depends: libc6
  Depends: libx11-6
  Depends: libxmu6
  Conflicts: xclip:i386