Options for authenticated feeds

Assuming you only want to switch it from excerpts to full content, then you could filter the rss_use_excerpt option based on whatever settings you prefer. Something like this would work:

add_filter('option_rss_use_excerpt','random_function_name_here');
function random_function_name_here($value) {
  if ( whatever_check_you_want_here() ) {
    return false; // false = show full content
  } else {
    return true; // true = show excerpts only
  }
}

The problem with a solution like this is that frequently the person getting the RSS feed isn’t the actual authenticated user. Google Reader fetches feeds without being logged in, for example. And it only fetches the feed once for all the users it’s showing the feed to.

So really, you need an entirely separate feed for members, with an entirely separate URL. This can be easily done as well, using the add_feed function. How and when you then display that separate URL is up to you and your own code.