the 202 is a newly designed keyboard which just came out with "current" features from some of the higher end products. There is a shuttle wheel (from the 550 and 2000 series) and the ability to provide any sound for the split you like.
The DGX300/500 is a new model featuring Piano type keyboard feel and looks but using the older PSR350 technology which doesn't allow for as many options as the 202 and was designed more for the Pianist rather than having a more complicated keyboard where you would have more options and more difficulty in assigning sounds, etc.
George Kaye
Kaye's Music Scene
Reseda, California