The Hyper X is Gunnar's disk based competition cyclocross bike. The Fastlane is an all around disk based cyclocross adventure bike. The Hyper X has the lighter tubeset of the two frames... which makes it fast and nimble. The Fastlane has a beefier set of Gunnar's OS2 Air Hardening Steel tube set, so it is more suited to carry larger cyclists, and more weight. You can certainly fit both frames with a carbon forks. It will give you a lighter, quicker response. Steel forks, however, have a unique sturdier feel to them, and soak up a LOT of vibration and road chatter. They don't have as nimble of a feel, though. Gunnar lists the Hyper X as being able to accommodate 700x35 tire with fenders. The Fastlane can accommodate 700x38 tires with fenders, 700x40 without fenders. Of course, which size of tire will depend on the wheel set that you chose.
Basically, if you intend to race competitively, the Hyper X would be the better choice. If you intend to hit the cyclocross path, but also want an all around adventure bike that can be equally as competent on the road as it is off the road, specially carrying some additional weight, then the Fastlane would probably be my recommendation.