It's a bit of both, resolution is definitely a factor that the higher quality models can truly boast.
As for comparison it's not so much a question of graphics as it is realism. The models I currently use tend to, for the most part, look "almost" like photographs of real people, whereas its predecessors, the lower-quality models tend to have that plastic and doll-like or cartoony in appearance.
The Ninetendo comparison is good, however, If you had to do a comparison, I would say a more accurate one would be comparing VHS quality to DVD quality.
In a nutshell, the higher quality models are MUCH, MUCH, MORE realistic looking than the lower quality ones.