They both are having different perspectives... and they both are important... .xml are one we create for SEs and submit to webmasters tool for their fast indexing (setting few thing like page priority, how frequently the pages being updated etc. so that SE can know.. which page should be given more importance in order of indexing )and .html are one which we place on our website for users and also SE robots... for SE robots it goes like this... if a robot lands your website and switch to Sitemap page after few clicks.. so that can pass through all important pages on the website... Its this simple.. if still there some confusion ... me and other fellows are here only for your help..
The answer is comprehensive enough to answer what has been asked for. Since you are asking about sitemaps, I would like to add that you can create the xml site for your website at http://xmlsitemap.com/ using the sitemap generator.
html site map is used to list all hyperlinks of different sections & pages of website. xml site map lists URLs for website in a special format.
html sitemap is website sitemap which is used for easy navigation for users. xml sitemap is for Google to easily crawl and index the webpages