Each section has distinct theming and advantages, and there is no real clear cut winner
Castias: Stately tropical theming with fountains and giant palm trees between the buildings. It's also real close to the main lobby (giving you a choice in bus stops), but far away from the pool. It also has the covered walkway to the lobby, as Moustronaut said.
Ranchos: The theming here is amazing. It looks like a barren Southwest desert, with boulders, small streams, etc. It's very quiet and secluded, and it will make you forget that you're in central Florida. It's also right next door to the pool, and a nature trail. The downside is that it is across the lake from the lobby / food court. If you don't mind the walk (and it is a nice walk), then I definately recommend this section.
Since you upgraded from the value resorts, I assume you want more than just a place to crash. So, I recommend Ranchos (I can't recommend any buildings since I'm not familiar with the specific layout). You can always drive from the Ranchos section to the lobby if you don't want to walk, but I really recommend the walk since the resort and waterfront are very nice at night.