The only difference between the 2700K and 2600K is that the 2700K is clocked 100MHz faster than the 2600K. Otherwise it's really no different. I would be in favour of getting the 2600K. It really depends on what you decide you'll need or want out of the motherboard and how much you're willing to spend. P67 and Z68 motherboards from Asus, MSI and Gigabyte are the dominant vendors I'd look into for a good selection.
No point of spending extra cash for 2700K, the same CPU's, if you want to achieve this 100mhz more on base clock, just change your bus speed to 103, and you will be done.
2700k are from a higher bin so should clock a little higher and run the same speeds on less voltages but a good 2600k will be just the same as a good 2700k you know, same chip after all.
Ivy bridge is due around April consider waiting for that. But if you don’t want to do that then go with 2600k the extra 100 MHz isn’t worth it. There is no grantee that the 2700k will oc better than the 2600k.