en_US
and en_US.utf8
is that the former uses ISO-8859-1 for a character set, while the latter uses UTF-8. Prefer UTF-8.
The only difference in these is in what characters they are capable of
representing. ISO-8859-1 represents characters common to many Americans
(the English alphabet, plus a few letters with accents), whereas UTF-8
encodes all of Unicode, and thus, just about any language you can think
of. UTF-8, today, is a defacto standard encoding for text. (Which is why
you should prefer it.)