Feed reader that allows subscription to only certain categories of posts from a blog

As you've noted, there are three categories that divide these up resulting in 8 different feeds:

15 vs 50 - This just tells you how many items appear in the RSS feed. Depending on how busy the group is and how often your RSS reader checks it, collecting only 15 messages could mean missing out on some if 20 had been posted. On the other hand, if the group gets very large postings a 50 item feed could overwhelm a basic RSS reader.

Messages vs Topics - Like you say, the difference between getting all messages or the first one of each thread.

RSS vs Atom - As a user there isn't really any difference between one or the other. Most of the differences only matter to someone programming the feed creation or reading process.

Have you looked at Google Readers 'sort by magic' feature;

http://www.gtricks.com/google-reader-tricks/what-is-sort-by-magic-in-google-reader/

http://googlereader.blogspot.com/2009/10/reading-gets-personal-with-popular.html

Only have a 10 minute coffee break and want to see the best items first? All feeds now have a new sort option called "magic" that re-orders items in the feed based on your personal usage, and overall activity in Reader, instead of default chronological order.

Hope this helps.