In general, I would say that if...
...you want to walk to DTD Marketplace or PI, You want Congress Park.
...you want to be able to walk to DTD West Side or the new pool/water play area or the golf course, you want Grandstand.
...you want to be close to the main pool, restaurants, and spa, you want The Springs.
...you want a quieter stay, you want the Paddock.
...you want a REAL quieter stay, and perhaps don't plan on walking anywhere, you want Carousel.
These all depend on how much you want to walk. You can get anywhere from anywhere regardless, its a matter of distance.
We stayed in a 2BR in the Paddock the first week of December 2007 (although we had asked for Grandstand or The Springs). I actually really enjoyed the walking, despite the fact that I easily get blisters on my feet.
The buses, at least in certain circumstances, were frequent, although I don't believe that all of them stop at every location.