Well I'll do my best to contribute even though I own neither
I believe the difference is that Ivy Bridge is more efficient with its architecture and is around 5% faster than a sandy bridge when at the SAME clock speeds. However due to the die size of the Ivy Bridge, it gets quite warm easily when you start to tamper with voltages (and overclocking). On the other hand sandy is better with dealing with voltage increases and so it can do a bit better at overclocks.
So from what I believe:
Ivy Bridge is faster (better) than Sandy Bridge at stock clocks.
Sandy Bridge is better at overclocking than Ivy Bridge.
PCI Express 3.0
22nm Die Process
Smaller architecture resulted in higher processor temperatures
Faster memory controller
Faster integrated graphics processor (IGP)
The Ivy Bridge processors are "newer" and support faster hardware. Clock to clock the IB chips are about a 200mhz difference in overall performance over the SB counterpart. So.. i5-2500k running 4.2Ghz is equal to an i5-3570k running at 4.0Ghz. The PCI Express improvement helps in SLi/Crossfire setups but single GPU platforms see very little performance increases. The memory controllers on the IB CPU's are a tad better as well when compared to the SB chips.
There is another, Sandy Bridge-E which supports mainly the Six-Core processors. However the performance gains with these processors isn't respectable, especially for the price difference and difference in socket types. SB/IB use the 1155 sockets while the SB-E chips use the 2011 socket. However, in most practical and gaming performance settings the higher end i7's of the SB and IB platforms are just as fast if not faster in some instances.