Preferred rooms at CSR are casitas 1, 2 and 3 - some of those rooms have a better location than others. All guarantee that you will be reasonably close to El Centro and the convention center. That might not meet the needs of some families, like ours, so we do not pay for preferred rooms. We prefer the cabanas to be close to the Dig Site, and like aubriee, we would pay a preferred room charge to guarantee cabanas.
Many people like casita 3 - it is lovely, and it is closer to the casitas bus stop (which is the first stop picked up and dropped off on the way to the parks). It is also closer to the Dig Site than the other preferred rooms, but farther from El Centro. If you are interested in the fitness center and/or the quiet pool (which is a lap pool), casita 3 is a great location.
If someone wants to be very close to El Centro, a request should be made for casita 1. If someone wants to be closer to the Dig Site, they probably shouldn't book preferred at all at CSR. Almost all of the standard rooms are closer to the Dig Site than the preferred rooms.
Popular standard buildings are casitas 4 and 5, cabanas 8A, 8B, 9A and 9B, and maybe ranchos 7B. Ranchos 6A, 6B and 7A don't seem to appear on many people's lists of favorites. My personal favorite is cabana 8A.
If I were to book preferred (which I don't since I like cabanas better), I would request casita 3. I think it is the best of the preferred buildings. Just personal opinion.