If you want the highest overclock with lower temps, the 2500K is ideal.
If you do video editing & want to use PCI-e 3.0, 3570K.
The 3570K can be overclocked high, and it does have an increase in performance, but if you overclock the 2500K (which can go higher) you can reach the same performance level as the 3570K on it's highest clockspeed, usually.
Ivy bridge is a ~5-~10% increase to Sandy bridge. I'm guessing the 2500k will last longer (assuming you are going to use it for more than 3-4 years) because it isn't as hot when overclocked meaning it will last longer.