According to Barry Schwartz's article about HTML and XML sitemaps, HTML sitemap "is an old school landing page for users to find all (or most) of your pages on your web site via single page" while XML sitemap "can also be multiple files, but they are not visible to a user, only a search engine spider."
Using HTML Sitemap both users and spiders can chew on it, according to Matt from Google when asked by the author which he would pick between HTML and XML sitemaps.
Here's the link to the full article:http://www.seroundtable.com/archives/020924.html
I hope this helps. Thank you.