The PS4 games will obviously look much better and be at a higher resolution, but I can't think of anything it can do game-play wise the Wii U cannot. The Wii U can render huge worlds, have dynamic lighting, locked 60 fps, have many enemies on screen, etc. The quality and resolution of the textures may be lower, and the lighting may not be as realistic but that's not necessary for an engrossing gaming experience, that's up to the director, developer, etc. Higher resolutions and textures won't make bad developers game's good. As well if you don't know how to make a believable world, more resolution won't fix that. To make a game/world believable that's in the art direction, writing, diecting, the experience, etc.
"Graphics is Power" - Nintendo Miyamoto.
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