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In all honesty I've never done any serious testing but I've always worked by a rule of thumb of 5 to 15 words max, with the keyword as close to the front as I can get it..

The thing to remember is that the HTML version should only ever be used for users and not to redirect robots if they hit a 404 on your .xml file. The reason for this is that search engines may still see the file as 404 after the redirect or a 301 redirect, if the later you then have an issue of search engines thinking it was there but is now the html page. Which of course is not a good thing. I would advise ensuring the fall back never happens to robots / spiders - if the file is just a 404 SE's will return to it, they may not if it is 301 redirect.