NIU is much closer for me to get to, but I prefer the environment at ISU because of the intimacy of the stadium.
NIU and especially U of I are too big. When ISU gave up the playoffs several years ago, the game-day experience waned.
Attendance at various IHSA state championships have declined over the years and a smaller college would be more advantageous as football host site. I even noticed some of the basketball super-sectionals are vacating college arenas for large high schools.
Truth be told, best atmosphere is at the high schools themselves, but that would never fly for a football state final.