People are easily forgetting the fact that Wii U has the ability to render two different complex scenes at the same time, as shown in this version of the tech demo with the sparrow and the falcon (the second part of the demo that wasn't shown on the press conference, looks really nice):
http://www.youtube.com/watch?v=i2Nsa06KRLo
The controller view of the scene is different from that on the TV.
My guesstimate is that Wii U will be at least 2x graphically stronger than the x360 and ps3. It won't be a mindblowingly substantial leap, but neither will be ps4 vs PS3 compared to PS2 vs PS1 or PS3 vs PS2.
Edit: I totally suck at embedding video, sou you lazy ppl will be forced to click on the link

.







