Nintendo has always known the difference between the playability and superior graphics. One is more important than the other. Games are to be played and enjoyed. The Wii provides just that: acceptable graphics with really fun games.
The PS3 does have some fun games but just like the 360, it suffered for lack of good launch titles.
In the future, the PS3 games will catch up to the system's abilities, and Wii's graphics will improve, too.