Most of Ivy Bridge's improvements were in power efficiency and integrated graphics. Generally speaking the two CPUs perform the same, the 3770k can't overclock as high as the 2600k, but is a little more efficient in executing instructions to make up for its lower overclocking ability.
Get Ivy if you plan on using the IGP for quick sync, as the HD 4000 on Ivy Bridge is a good deal faster than the HD 3000 graphics on Sandy Bridge, or you want to have PCI-E 3.0 support, which may be useful down the road when you want to upgrade your graphics card.
If you don't really care about those two features, a Sandy Bridge i7 is still a good buy. Also note that if you are gaming, there is no point in getting the i7, aside from bragging rights. Virtually no games take advantage of the i7's hyperthreading.