“Best”, as in safest, or “best” as in smallest file? That's why there are two options ;-)
These are really a remnant of old Postscript drivers and printers that could not handle raw binary bytes spoon-fed to them. Even then though, it was usually not a problem, and the last decade or so I've been using Binary with no problem at all.
(Background: this sets the output — storage — format for all data streams; not particularly plain text, more stuff like fonts and images. If you choose “ASCII”, the binary contents of these files are written to the output file in hexadecimal format (0123456789ABCDEF …), or a modern equivalent such as ASCII-85. That's to ensure the receiving party — a printer, or perhaps some really backwards mail protocol — will not try to 'handle' binary codes as End of File (Ctrl-Z, dreaded by programmers ever since the Days of DOS) and end-of-line codes (Carriage Returns that get converted to CR/LF pairs, or the other way around).
Binary is just that: the literal binary bytes, Ctrl-Zs, CRs and LFs and everything else you can throw into a binary file.
Nowadays, the receiving end can't ever get confused of what's what; both types of data are clearly and loudly announced in the file before the actual data stream starts.)
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