If you submit XML sitemap, it will be easy to index your WebPages. And if you submit html sitemap ,This allows visitors to navigate your site easily.
By submitting your xml sitemap, you will notify the search engines about your website and index every page of it. But sitemap.html give an idea about all the web pages, to the visitor of the website. XML is for Google, HTML is for Google AND users. You should use both, as .HTML is also good for internal linking.
do not understand the purpose of sitemap and googlebot anyway crawls and indexes the websites http://www.crawlcraft.com
I have both on my site at present, but would welcome ideas on how often to update them with Google webmaster?