I don't know about the ranking being better or not. But one advantage of having an .html sitemap linked from your homepage is that it ensures that each page listed on your sitemap is only a maximum two clicks away from the home page. That is good SEO.
What Ive seen that If you have a "sitemap.xml" file and submit to Google webmaster tools that your site will get indexed very quickly. Jim4767 is perfectly correct also and I use Joomla , and a component called Joomap is available which produces a very nice Sitemap in Html and also in xml format for Goolgle ( and Yahoo also..) Cheers
sitemap is only a list of your web pages. you will give the list of your web pages to google. if google is crawling your site regularly and indexing your site, it's not urgent for you
site map is used to let google bot understand your site structure easier, so that it maybe get the information from your site much better tahn you don't do it. It like when someone told you to go to Store and buy something you may spend a lot of time. By the way if that person write the wap for you can tell you where is the location of the stuff you have to buy ,it will reduce your time right? sitemap is not the way to get better ranking in google,but it will help get index from google easier . Then it have a lot of factor to consider to calssify your page rank suck as your back link, your index and more. If you ask me, all i can say is it will help in the indirect way but not much