Based on my personal experience, I have not seen any direct correlation with any sitemap and improved rankings. I have seen some improvements in indexing. I have not seen significant differences bewteen txt or xml benefits. I do prefer xml since it allows me to input more data.
Look at the content of the two files. Both Google and Yahoo can handle text and XML. If your XML sitemap file only contains URLs, then it really only contains the same data as a text file listing all URLs. Thus there should be absolutely no difference :)
You might want to check this article for more explanation and examples: [microsystools.com...]