Being from the area, I can only comment on NIU, Valpo, and SLU.
First of all, you have to consider that DeKalb is probably 60k WITH the students. Pretty much the University is the town. It is in the middle of nowhere, but is only about 45min-1hr from downtown Chicago (in decent traffic). The cold weather would probably be a shock, today is the first time in a couple weeks it has been over 20 degrees in Illinois.
Valpo is even smaller, and is really a bastion of affluence in an dingy area of Northwest Indiana. The nearest town that has any sort of business is Merrillville which has a population of about 40k. The cancer that is Gary is expanding southward and slowly making Mville into a slum. Valpo is far removed from this though. There are a lot of Valpo lawyers in Mville, and from what I understand it is exceedingly hard to get your foot in anywhere else because of the law school's reputation. You would have a hard time getting into Chicago because of the stiff competition there, and also in Indy because of IU.
NIU is very young so it doesn't have a very established alumni base, but it does travel well throughout non-Chicago Illinois. I think you would be better able to get a govt job in IL with NIU than Valpo. Between those 2 I wouldn't even consider Valpo.
SLU is much much better. It is great for the St. Louis market. St louis weather is a little more mild. The concern that I would have is the cost.
If you were set that you wanted to live in a larger city, I would take SLU, no question. You would just be burdened by debt. If you dont mind living in a smaller city in Illinois, then NIU could be a great option.