Both are useful. Both sitemap are important but both sitemap's working strategy are different. HTML Sitemap - This is useful on user point of view, with the help of this user can navigate in whole website. XML Sitemap - This sitemap is useful as per Search Engine Point of view, XML sitemap help search engine to crawl every page from there. When Search Engine crawl your website then Sitemap will help Search Engine to crawl all page of your website.
Both the sitemap are important and has different use.We create xml sitemap for search engine crawler xml sitemap has all links of website which is crawled by search engine submission and HTML sitemap is developed for the use of user
both have their own value, HTML sitemap is used for User point of view and whereas XML is used for Search Engine and spiders point of view just to read your site pages and index them.