3DS is mostly weaker. CPU wise it's not even a contest, only high-clocked Cortex A8 or better ARM CPUs can compete with Wii's PowerPC. GPU wise, 3DS has the edge on shaders thanks to the newer OpenGL ES fixed-function implementations of the GPU, however, it loses in polygon count and texture quality.
A great comparison would be Monster Hunter Tri vs Monster Hunter Tri Ultimate on 3DS. Since both are technically the same game, you can clearly see the compromises 3DS had to make in order to be able to play the game smoothly. Mainly, textures have much lower resolution on 3DS than on Wii and polygon count has been reduced on the background.
3D certainly uses a lot of 3DS's power, though. Disabling it should get the 3DS graphics much more similar to Wii's, since when the 3D isn't being used games can actually use 2x2FSAA without a performance drop.
