UIUC is no city and that's probably it's biggest weakness, but as a PhD candidate in Material Science, you also have to consider how much time you'll have for a social life in general. This is not to go with the cliche that all graduate students do is study, but I still think it is pretty much one of the main defining differences between the two.
I've been at UIUC for 4 yrs and it definitely has it's charm; when I go crazy for the city, I'll drive up to Chicago now and then to breath a little. It's not a place I could see myself settling down in, but I've already considered myself just a temp visitor, hehe...ask yourself what your priorities are (under the conditions that both are already HIGH priority): social life > academic opportunities OR academic opportunities > social life and see where Minnnesota and UIUC fits.
If you have any questions with regards to champaign life, etc. I would be more than willing to answer them