Hey Travis...I am an IL resident who has talked to some of the faculty at SIU-Alton (where dental school is) and might be able to answer this one.
Basically, what SIU has told you is true, they rank consistently in the top 10 on the NBDE (National Board Dental Exam) which you take after your second year (at most dental schools) and after your fourth year (correct??).
After talking to the prosthodontic department head, he mentioned they always score in the top 10 on the NBDE's because they place an enormous emphasis on studying and preparing for it. Also, unlike some schools where you are immersed with med students, at SIU the faculty is there purely for you (these are his words, I am not elaborating here). Some schools also care more about you finishing a certain amount of work in the clinic, or being in the clinic (clinic attendance) for a certain percentage of time (not necessarily a bad thing) while SIU gives their students lots of preparation and time to study for the NBDE. However, the important idea in all of this is that on the NBDE a pass score is required...a high score will help you if you want to specialize but wont make you a better dentist. All you need to do is pass (granted 'merely passing' isnt all that simple).
In any case he also mentioned something about being #1 on the boards in terms of the midwest region.
In the end, however, this stuff doesnt matter. There are no rankings and all schools have their +'s and -'s. Thats just it...the point is to find out if you could see yourself enjoying your time the most at a particular location to determine where you want to go (as well as other factors such as size of clinics, class size, amount of patients in the area, what the clinics look like, overall feeling of the students, the way the curriculum is set up, and many other factors.)
Dont just go by what SIU tells you. SIU is a great school (dental school) and I am sure you will be fine. Try not to get caught up in the numbers game, or by those who use SDN as their own comic relief. Be confident in your choice.