You keep saying this but no they really don't, if you look carefully you can see major differences:
-The 3ds model is not as smooth as the Wii U one.
-The platforming (like the clouds for example) have some sharp edges whereas the Wii U smoothens the edges for platforms that are suppose to mimmick soft a partially round objects.
-The 3ds enemies look partially blocky. Whereas the Wii U smoothens the surface of the model.
-The graphics for the 3ds version is almost like a tiny improvement of the Sonic Colors DS graphics.
-The Kingdom Hearts 3D Dream Drop Distance graphics are more improved than the Sonic Lost Worlds graphics on 3ds.
The 3ds lacks the smooth textures and makes the Sonic model look slightly blocky, it is very obvious.