Concrete slabs tend to be really cold and uncomfortable during winter. They are common in tropical areas exactly because they keep the house cool during scorching tropical summers.
Therefore, if you go with concrete slabs in IL, you will want a radiant floor.
Crawl spaces are said to be inexpensive but as far as energy efficiency, they are really bad, unless they are encapsulated and conditioned. Studies show that energy losses from vented and dirt crawl spaces can be as high as 35%. Even more (up to 50% more) than that if you have ducts running through your crawl space.
Not to mention moisture problems, mold, mildew and pests. In other words, if you pick a crawl space, have it encapsulated and sealed right away.
A basement, as Triple D suggested, is a bit more expensive to build, but is adds value to your home, and provides additional living or storage space. Spend some money on the best perimeter drainage and waterproofing solutions you can find to avoid problems on the long run.