When comparing 6x13 fixed vs Apple San Francisco Mono, the Slant community recommends 6x13 fixed for most people. In the question“What are the best programming fonts?” 6x13 fixed is ranked 37th while Apple San Francisco Mono is ranked 38th. The most important reason people chose 6x13 fixed is:
6x13 fixed is the apex of bitmap fonts. It has a nice, defined character set and great proportions for quantity of text onscreen, making for excellent readability. It is aesthetically pleasing for a bitmap font and has been deservedly termed 'classic'.
Specs
Ranked in these QuestionsQuestion Ranking
Pros

Pro Well designed
6x13 fixed is the apex of bitmap fonts. It has a nice, defined character set and great proportions for quantity of text onscreen, making for excellent readability. It is aesthetically pleasing for a bitmap font and has been deservedly termed 'classic'.
Pro Legible at small sizes
At a small text size, each character has a limited resolution. A character size of 6x13 pixels means only 78 pixels per character. Modern fonts are designed to be scalable and are less legible at these small sizes. Using bitmap fonts increases legibility by eliminating scaling and sub-pixel aliasing artifacts. Some scalable fonts include "ppems" embedded bitmaps for this reason.
Pro Available on every X server
6x13 is the classic fixed monospace bitmap font that is expected to be available on every X server. It is part of the misc-fixed family. These fonts were handcrafted for readability in a terminal.
Pro Widely available
It is distributed alongside the X Window System.
Pro Clear and dis·tin·guish·a·ble
Code is very readable at both big and small sizes.
Pro Beautiful with high pixel density
Pro Best legible font
This font is legible in every condition.
Pro Visible punctuation
Punctuation is prominent and easily visible.
Pro Looks good on low and high PPI displays
SF Mono looks good on external monitors and Retina Displays. Large and small font sizes.
Cons
Con Doesn't have a slashed zero
The absence of non-slashed zeroes makes it harder to distinguish "0" from the letter "O".
Con Extremely small on high-DPI screens
While it is crafted for a screen where the pixels are visible, bitmap fonts do not work well on high-DPI screens as they do not scale too well.
Con Apple is locking down this font
It's not open in any sense of the word. It even gives warnings if you try to rip it out of the Terminal.app or Xcode bundles. Obviously, Apple only wants it on their tools. This is such a shame. It should work in other editors, too. It's a beautiful font. Apple open sourced swift, why can't they be open with a monospace font?
Con No Ligature support
Does not support Ligatures so it doesn't play well with some terminal themes.
Con Lowercase l (ell) is similar to 1 (one)
Con Slightly difficult to use outside of Xcode, Terminal, or Console
The typeface isn't available in Font Book, etc. unless the user imports the files embedded in the apps above.
