Extreme4 P67 vs Extreme3 Z68

http://secure.newegg.com/WishList/PublicWishDetail.aspx?WishListNumber=13328589 is my build. I will buy the i5-2500k at microcenter for $180+tax (I live in Texas). I currently have an ASRock Extreme3 Gen3 Motherboard on my newegg wishlist. I saw that at Microcenter, the P67 Extreme4 Gen3 ( http://www.microcenter.com/single_product_results.phtml?product_id=0373371 ) is $100+tax. Is there any difference between Extreme4 and Extreme3? If so, should I get P67 or Z68? I won't be using SSD Caching and I heard P67 overclocks just as well.

Thanks all!