Ivy Bridge (ix-3xxx) is just a die shrink of Sandy Bridge (ix-2xxx). Except for the integrated GPU, a few new hardware functions, and some bug fixes, they are the same. Ivy Bridge is just manufactured on 22nm while Sandy Bridge is on 32nm. So clock for clock, the two will perform almost identically (outside of the GPU, a few now hardware functions, and bug fixes).
The i5-3570 has a 3.4 GHz base speed, and will turbo boost to 3.6 GHz on 4 cores, 3.7 Ghz on 3 cores, 3.8 GHz on 2 or 1 cores. It only has 6 MB of L3 cache, and does not support hyperthreading.
The i7-2600 has a 3.4 GHz base speed, and will turbo boost to 3.5 GHz on 4 cores, 3.6 GHz on 3 cores, 3.7 GHz on 2 cores, 3.8 GHz on one core. The only difference in clock speed with the i5-3570 that it is 3.5 GHz on 4 cores, while the i5 will hit 3.6 GHz. The i7 has 8 MB of L3 cache, and supports hyperthreading.
So unless you're going to run games on the Integrated graphics, or are concerned about power consumption, the i7-2600 is a better choice. The larger L3 cache and hyperthreading should help more than an extra 0.1 GHz when running on 2, 3, or 4 cores (about 2.7% faster). How much more is it worth? That's something you'll have to decide. We can't tell you since we don't know how much money you have and what you'll be doing with this.