http://www.w3.org/TR/xhtml1/#C_16 states:

The [XHTML spec](http://www.w3.org/TR/xhtml1/#C_16) states:

"C.16. The Named Character Reference &apos;

The named character reference `&apos;` (the apostrophe, U+0027) was introduced in XML 1.0 but does not appear in HTML. Authors should therefore use `&#39;` instead of `&apos;` to work as expected in HTML 4 user agents."

While `&apos;` is more human readable, it will be ignored by some clients, so it seems best to ignore it completely.