When comparing Proggy Clean vs Fira Code, the Slant community recommends Fira Code for most people. In the question“What are the best programming fonts?” Fira Code is ranked 1st while Proggy Clean is ranked 42nd. The most important reason people chose Fira Code is:
This is particularly beneficial for those who wish to use combined letters such as "æ" and other diphthongs. But when it comes to programming, the ability to scan through your code is improved with ligatures for equality, arrow functions, and more.
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 Has ligatures
This is particularly beneficial for those who wish to use combined letters such as "æ" and other diphthongs. But when it comes to programming, the ability to scan through your code is improved with ligatures for equality, arrow functions, and more.
Pro Supports retina displays
Fira Coda supports high pixel density retina displays.
Pro Characters look really nice
Some characters that look odd in other monospace fonts look very nice in Fira Code: @, a, 1, lower-case-L, Q, j, *
Pro Good editor support
A list of supported editors and terminals can be found here.
Pro Has a slashed zero
New style since February 2018.
Pro Frequent updates
The repository is frequently updated.
Pro Installs easily on Mac
Many ligature fonts on Github aren't "mac ready". This font comes pre-compiled and ready to install on Windows, Mac, and Linux.
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 The '@' Symbol is asymmetric
It's a style, but it would be nice if it would wrap and not just cover the top.
Con No true italic
Italic is just a slanted original, an Oblique. Looks ugly and is difficult on the eyes.
Con Noisy serif-like style harming the text clarity
Con Ligatures are nice-looking but harm clarity
Even though the font combines characters into ligatures, you still need to type the normal characters, and the ligatures make that difficult in many cases.
Con Ligatures like == and === are harder to tell apart than they should be
Con Too wide, too large line height
Con Too wide
Much wider than other fonts.
Con Ligatures lump some characters together and make them hard to read
Con Needs support for ligatures
It can't work in plain terminal, must have built in support for ligatures in editor.
Con No Sublime Text support
Not the font's fault but even the latest Sublime Text builds (e.g. 3126) don't support ligatures.
Con Ligatures break correlation between symbols on screen and the number of characters
This makes it easier to lose the grasp how long lines actually are.
Con Curly braces are not clear enough
Curly braches ("{" and "}") are not clear enough. They are too horizontally narrow, making them look almost like pipes ("|").
Con Bad 4 and r characters, dotted 0
WHY is r a serif?
Con Cannot enable alternative stylistic styles on Xcode
I've tried enabling some of the alternate stylistic styles using Xcode's Font picker, via the "Typography" screen. None of the stylistic styles I enable get reflected in Xcode's code editor, even if I restart Xcode from scratch. I'm not sure whether this is a limitation of Xcode, or of the font itself.
