If you want real quick indexing and good SEO, sitemap is the basis of you SEO journey! I prefer XML, the best plugin for sitemap is Google XML Sitemap Generator. if you have any questions PM me !
One of both is best. For sure an xml sitemap, and then a separate, human readable sitemap is what I do.
Plus 1 for XML as it is best read by Google webmaster central and get a good response from the searches..
Comparing one with the other is not an wise act. Both sitemaps are needed and are important in their places. While HTML sitemap is for users and is needed to enhance the user experience, the XML - on the other level - are important for Google. It helps Google to find and index links faster and easily.
I would also go for the XML sitemap. If you own a wordpress blog, you can use the "Google XML Sitemaps" plugin.