Both are used for Search Engine Optimization. But mostly HTML sitemap is used for Users because they can navigate easily on the website from sitemap. Same thing we can say with XML sitemap, when search Engine crawl the website then search engine can easily track all pages from the XML sitemap.
sitemap is a way where search engine like Google read crawl website page easily. there are two type of sitemap 1). HTML sitemap :- HTML sitemap is used for Users and 2). XML sitemap :- you enable Search Engine crawlers (like Google) to find out what pages are present and which have recently changed, and to crawl your site accordingly.
Xml sitemap is created and submitted in search engines so they easily crawl our websites.By using these search engine easily know the every webpages of our website .Sometime lots of url is not discover by normal crawling process so we submit the xml sitemap to search engine. Xml is only for search engine. HTML sitemap help search engines crawl and understand your site, and it also helps human visitors browse your site more efficiently.Generally xml website is used to submit in webmaster tool so it is more famous than html.if you want read more than use this link http://www.seroundtable.com/archives/020924.html
If you have mini site then there is no need to use any sitemap. Sitemap is for large websites... The main benefits you get: xml sitemap: content indexing html sitemap: easy navigation
Mainly we use .XML sitemap for our website. Html sitemap only shows the data but XML sitemap stores and transfer the date.