Welcome!

We have stayed in the Casitas and Cabanas and like both. We were in Cabana 9A, which was close to the Dig Site and also close to El Centro (the main building). Great location, and my kids enjoyed swinging in the hammocks in the cabanas area. There is also one hammock in the Casitas, but since you are going in September, you will probably find it occupied by Kathy (KatMark)!
The casitas have beautiful fountains and are located near the first bus stop. It is still close to the main building but a bit further to the Dig Site.
We have not put in a request for our next trip, but I am kind of hoping for Ranchos since we haven't stayed there before. We will have a car in case we are too lazy to walk 10 minutes to El Centro at the end of July!

The Ranchos are the farthest away from the main building and are the quietest (although the whole resort tends to be quiet). People have a love/hate relationship with the Ranchos - they are beautiful and quiet with a nice quiet pool, but it is a hike the the main building (not compared to walking in the parks all day, though!).