What are the differences between Ivy bridge and Haswell
Any performance differences in gaming
Motherboards for ivy bridge tend to be lower in price than Haswell
Which should i buy?
I usually use this site to compare CPU but the results are not always enough for me to decide (LINK)
The main focus of Haswell to bring down power consumption for devices such as laptops and tablets. Desktop only sees slight improvements for gaming, but nothing that would be a significant difference. If you want go for the Ivy Bridge, or maybe wait for the Haswell refresh to see if there is any improvements.
Go with the 4670k Haswell. While the performance increases aren't anything dramatic they are there and every little bit helps.
Plus, by going with a 4670k you are going with a newer chipset (Z87 or Z97) and a new socket (LGA 1150 instead of LGA 1155). By doing that you get a lot more features like more USB, more SATA, M.2, and lots of other niceities that come with the new chipset. Additionally if you want to upgrade to Broadwell in the future you won't need to change the motherboard as Broadwell will also use the 1150 socket.
Z87 motherboards are actually cheaper usually. That is especially true when you take into account how much more you are getting on a Z87 board as opposed to a Z77 board.
Now, when it's so far into the LGA1150 socket, I would go with the 4670k. If it was when Haswell released, it may have been a different story. the i5-3570k is still a good processor, but it's losing value as it becomes "old silicon".