Actually, it's not. The Xbox has Direct3D shaders which were easy for devs to exploit but the Wii has more horsepower overall.
From what I understand, basically...
Xbox: D3D compliant, more programmable/flexible, HDD caching
Wii: faster CPU/GPU, more memory, far less latency, WiFi
...they're generally "on par" though, like comparing 360 and PS3.