The Gen 3 mobo can do PCIe 3.0 which is good if you upgrade to IvyBridge when it arrives and buy a PCIe 3 GPU. Othrwise it is the features specific to the mobos.
Here you go: Read this. Are you buying today, or waiting for Ivy Bridge? If you are waiting, go with Z77.
On the other hand, if your are purchasing this weekend, you will realize about a $20 savings on the Z68. I'm spending the savings on a graphics card upgrade.