You can make a request when you book the room. Casitas 1, 2 and 3 are preferred rooms, so they are a separate booking category.
Pros/cons of each section:
Casitas are closest to the main building and convention center. The casitas bus stop (stop #2) is the first stop picked up and dropped off to/from the parks. I think the casitas are the prettiest and most romantic. They are a bit farther to the main pool/hot tub/arcade/volleyball court. The fourth floor of casita 2 is (somewhat) affectionately referred to as no-man's-land. Casita 2 does not have its own elevator - it shares one with casita 1 (which also has another elevator near the convention center). The elevator doesn't go to the fourth floor.

So you can either walk up the stairs from the third floor or take the casita 3 elevator to the fourth floor and walk over. Strange and annoying, but probably only a real problem if you have a stroller or mobility problems. Casitas 4 and 5 are great non-preferred locations.
Cabanas - our personal favorite, because we travel with kids and spend more of our resort time at the Dig Site pool than at El Centro. Sandy beaches, hammocks (you'll also find hammocks near the casitas), very close to the pool, and the third bus stop picked up on the way to the parks (stop #4). Can be very close to pool, very close to El Centro, or centrally located.
Ranchos - most remote, quietest. Can be close to the pool or far away from everything. Long (ten minute) walk to El Centro. Bus stop #3 is the second stop picked up. Rancho 6A is close to the bus stop but far from everything else. Rancho 7B is close to the Dig Site, Ranchos quiet pool and laundry. 7B is not a bad location. 7A and 6B can be sort of close to the Dig Site and/or bus stop, but it depends where you are in the building. Ranchos is best if you want a very quiet area and don't mind walking.
You can make your request at booking, and you can request a change at check-in if you don't like your area and they have something else. On one trip, we were assigned casita 2, but we switched to cabana 9A because we wanted to be closer to the pool. 8A is my favorite building - very close to the bus stop and very close to the Dig Site.
This map may help:
Happy planning!
