At the same clockspeed Ivy Bridge is on average 6% faster than Sandy Bridge.
Ivy Bridge consumes a little less power than Sandy Bridge. I forget the exact amount, but at 100% load I think it is around 6w less.
The biggest improvement is the integrated GPU (iGPU). Ivy Bridge has the Intel HD 4000 while Sandy Bridge has the Intel HD 3000. In desktop CPUs, the HD 4000 is about 35% more powerful than the HD 3000. The desktop Intel HD 3000 is basically equal to the Radeon HD 5450, but it lacks DX11 support. The Intel HD 4000 has about 85% the performance of the Radeon HD 5550 and it does support DX11.
But in laptops the performance difference between the HD 4000 and HD 3000 is not so cut and dry. Depending on which Ivy Bridge / Sandy Bridge the laptop comes with the Intel HD 4000 / HD 3000 runs at different frequencies. The mobile CPUs found in ultrabooks will have the slowest performing iGPU to save power (as low as about 850MHz for the Intel HD 4000). The quad core mobile i7 "QM" CPU will have the highest performing iGPU (running at 1.3GHz for the Intel HD 4000).