The Extreme4 has more SATA3 & USB3 ports. It also has an extra PCIe 2.0 x16 slot.
The Extreme3 has more USB2 ports.
Here’s the link for the comparison chart: http://www.asrock.com/mb/compare.asp?SelectedModel=Z68+Extreme4+Gen3&SelectedModel=Z68+Extreme3+Gen3
Thanks a lot man, that chart really helped me. I decided to go with the Extreme4 considering that most of my future products will be USB3.0. Future proofing, ya know?