Here's my perspective after having followed both paths:
System.ServiceModel.Sydication
- is about 9 times faster than Argotic
- does not have rdf support
- doesn't have support for atom versions < 1
- doesn't have support for rss versions < 1
- fails with XmlException on about 30% of the feeds I give in "from the wild" (this is the deal-breaker for me)
Argotic
- hasn't been updated since Jul 2 2008
- is slow (see above)
- has very good support for all types of feeds: all version of atom, rss, rdf (as far as I can tell)
- is slightly harder to use than System.ServiceModel.Sydication; you need to determine what kind of feed you have before letting Argotic parse it, and then you need custom logic to deal with each of 3 different classes depending on the type of feed. In other words, you need to wrap Argotic before it's useful.