It seems 343 is jumping on the pre-order exclusives bandwagon with an array of weapon and armor skins for Halo 4. It was also specifically noted that these will not affect the gameplay in any manner so you will not be at any advantage or disadvantage by picking one over the other, or none at all.
Not all of the exclusives have been announced by their respective retailer, but below are a collection of images that will give you an idea of what will be available.