Wheat is dried barley, oat, etc, which is usually something that should be kept to a minimum in terms of consumption as it has no nutritional value to a rabbit. That being said, I used it for bedding for my rabbit before and she ate very, very little of it; I used it because it was easily accessible for me and she strictly likes her hay, not straw. Wikipedia article for Straw reads:
"Straw may be fed as part of the roughage component of the diet to cattle or horses that are on a near maintenance level of energy requirement. It has a low digestible energy and nutrient content. The heat generated when microorganisms in a herbivore's gut digest straw can be useful in maintaining body temperature in cold climates. Due to the risk of impaction and its poor nutrient profile, it should always be restricted to part of the diet. It may be fed as it is, or chopped into short lengths, known as chaff."
Explains it far better than I can!
As for hay, there's a lot of kinds out there and timothy 2nd cut is the best but not all rabbits will eat it. My boy won't eat timothy but he'll eat orchard grass. So you kinda have to let your rabbit decide, in a way, and pick what's best from their list of OK hays.
Some people also mix the hay up and make it "forage" by getting several types and mixing them up together so the rabbit can paw through it all and find which bits they like best. From what I've seen, some people mix orchard, timothy, botanical, oat hay, etc for such forage hays.