They are not alternatives.
ASCII is a subset of Unicode with code points 0.. 127. Just use Unicode. Make sure the file is saved in UTF-8 (could be any other UTF; they all support full set of Unicode code points, but the standard is UTF-8, other UTFs are impractical). By the way, unlike other UTFs, UTF-8 is designed the way it presents the characters withing ASCII subset exactly in the same way as ASCII encoding, with exact same bytes (and with one byte per character), other code points are encoded with variable number of bytes.
Its important that the HTML document has matching
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
It can work even without
, but there are situations where the lack if it can create some confusion. One example: a site can use UTF-8 by default and always claim this encoding in HTTP header, but if the page is saved as a local file, the local system does not "know" about it.