Like comparing apples and oranges. A google sitemap helps get more of your site pages indexed. An RSS feed can build a following of regular readers and adds another dimension to your link and traffic building.
sitemap is used for the Search Engine to know about the pages available on your site. RSS feeds is used to check new content. Having new content helps Search Engine to visit your site often.
RSS is for short summary of your site ... where sitemap is to show search engine that do follow pages on that site ...
RSS is alright but really you will not benefit very much by it. The sitemap is however quite beneficial and it can help all your URL's to get indexed by Google
Google site map is more important in term of SEO. But helps of RSS you can generate more traffic from other site. So used both method for your site. Thanks