When comparing Proggy Clean vs Liberation Mono, the Slant community recommends Liberation Mono for most people. In the question“What are the best programming fonts?” Liberation Mono is ranked 17th while Proggy Clean is ranked 42nd. The most important reason people chose Liberation Mono is:
Liberation Mono works well with hinting on high-resolution displays.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro The easiest to read
All characters are completely readable in the smallest size.
Perfectly usable in terminal and code editors in 12px text size.
Pro Hipster among hipsters
Even hipsters will marvel at this font, praising you as the new hipster lord. It is best at around 19px (tested on Mac). Overall, it has better readability than many default fonts.
Pro Very readable
Liberation Mono works well with hinting on high-resolution displays.
Pro As of 2023: Second Best screen-modern drop-in replacement for MT Courier New
Metric-compatible with Monotype Courier New, dot 0 (zero), modern non-Kettler lowercase l (ell), 660+ glyphs, OFL-1.1 license
Pro Compact line height
Just like Courier New (the metrics of which this font emulates), its line height isn't too big, which results in more lines on the screen, compared to Consolas and several other more popular fonts.

Pro Has Bold typeface
Cons

Con Pixelated
This is, of course, intentional; but it can be hard to look at compared to other smooth mono-space fonts.
Con Smallest font sizes hard to read
Fonts less than 11pt start to fail readability tests. 9pt specifically has an additional issue where brackets don't align.
Con Not really good in all font-sizes
Completely clean pixels, no anti-aliasing so its best viewed in (n*12) pixel size.
Bold font-weight is not really good because of small padding between them.
Con Difference between "0" and "O" isn't clear
It can be difficult to tell the difference between the number "0" and the letter "O". However, the Fedora Project has a slightly altered version of Liberation Mono that fixes this with a dotted zero.
Con Liberation fonts are ugly
