I'm just about finished with my junior year at AU, so here's what I know:
I applied and was admitted to both AU and NIU, but decided to go to AU for a couple of reasons. First, NIU's nursing program is five semesters long (and I had been admitted for spring 2009, so it would have taken me a full year longer to graduate from NIU.) AU costs quite a bit more, but I figure that extra year of working will more than make up for the difference in cost. Second, I graduated from NIU in a different field, and just couldn't stand the thought of five more semesters in DeKalb.
I seem to remember hearing that AU admitted about 1 out of 3 applicants when I applied, but I don't really know if that's accurate. I also heard that NIU admits about 1 out of 10, so AU would be statistically easier to get into. Both schools accept transfer students into their nursing programs, so you could apply to either (or both) regardless of where you go for your first two years. Again, NIU would be the low-cost way to go, but their introductory courses can be large and impersonal. I took my prereqs at a community college, and I was pretty happy with it, especially the A&P courses (two semesters, 8 credits, with cadavers - NIU crams the A&P into 1 semester, and I don't believe that course uses cadavers). I don't think AU has cadavers either, but their intro classes are pretty small, as far as I know.
As far as the actual nursing school is concerned, I can only discuss AU. I don't think anybody actually enjoys nursing school, but I can't really complain about AU too much. Most of the instructors are good, and several of them are outstanding. There are only eight (I think) full time faculty, so you'd have at least one class with just about all of them by the time you finished. Clinicals are mostly at local hospitals, like Rush-Copley, Delnor, Mercy, CDH, etc., except for Peds, which tends to be in Chicago (Children's or Shriner's). I haven't ever run into any NIU students at clinicals, but I imagine there's some overlap.
I suspect one could land a great nursing job after graduating from either program, especially if the economy has improved by the time graduation rolls around. 100% of AU grads passed the NCLEX last year, and their passing percentage is always in the high 90s or better. NIU might have more of a reputation, if that's a concern, but I don't think you could go wrong either way.
Hope this helps - feel free to ask anything else, but I'm new and can't do PMs yet.