Are you planning on using your vocal degree for your career or your business or whatever other degree you also plan on receiving? NU is more on the radar for music than UI even though Illinois certainly doesn't have a bad reputation. When you're auditioning for grad schools for music they want to see where you're going more so than where you've been, so the most important thing is how well you sing, not where you got your piece of paper from. Basically, I'd look at who you'd be studying with, especially if you want to make music your career, and make a choice that way. At the same time, if you don't plan on using music for your career and you just enjoy singing to the point of wanting to pursue a music degree along with another one, it may be wisely frugal to go for the cheaper school. Then again, I don't know anything about the voice faculties at either institution so it's up to you to determine if the school with the better name is actually the better one.