I'd get a second opinion on that frame size- it might be right, but sounds small to me for a six-footer. I ride a 54 at just over 5'10" with long legs. Is this mostly a race bike? The fork material doesnt matter much, stick with the cheaper Aluminum fork. The bumps on a cross course are big enough that it really doesn't matter much which fork you use, you can't really feel the diff, plus your big squishy tires make it all moot anyway. There's a little difference in weight between an Al fork and high end carbon but its an expensive way just to shave a little weight. If you are planning on riding mostly on the road, a carbon fork may feel a little more comfortable over a long day than Al, but a carbon cross fork is tons stiffer and less compliant than a carbon road fork, so its going to feel harsh.