For years, I’ve been using NewsFire as my RSS reader of choice, because of its intuitive, elegant interface, but I recently made the switch to NewsGator’s NetNewsWire instead, for 3 reasons:

1. The three-pane interface – As nice as NewsFire’s implementation of a 2-pane interface is, I still like to view the available articles at a glance, because, for example, I don’t read every single post on Slashdot; I like to pick and choose what I read.
2. Easily customizable post view styles – You can write custom CSS and markup to view items from the feeds in both apps, but in NewsFire, it’s inside the app contents, whereas in NetNewsWire, all custom themes (and there are many) are available in the ~/Libary/Application Support folder. The drawback to how NewsFire does it is that your styles get wiped out every time you upgrade the app. Not so with NNW.
3. Syncing – I have two laptops, and I couldn’t sync my feeds or read items across them except by manually exporting from one client and importing into the other. NNW syncs across either NewsGator’s own server, or through MobileMe. Now I always have my most recent feeds, whichever computer I’m on.

Other NNW features that are nice but not necessary for me include the built-in browser and the ability to blog an item you read in your feed.