We have heard developers saying that Wii U is 50% more powerful than the PS3 and Xbox 360 a few months back.
It seems those estimates were more based on specifications than actual performance. A source with the Wii U Daily has revealed the specs of the Wii U:
-Quad Core, 3 GHz PowerPC-based 45nm CPU, very similar to the Xbox 360 chip.
-768 MB of DRAM “embedded” with the CPU, and shared between CPU and GPU
-Unknown, 40nm ATI-based GPU
It also mentions that Nintendo is testing two version of the console, one with 768 MB of RAM and the with 1gb.
These specs might not be the ones we get when the console is release as it still has one year to go before release.