As an undergraduate, I spent most of my time taking required classes, but around the end of my degree, I took a few grad-level courses, which I can certainly say were enjoyable. The system in the CS department in UIUC is as follows: there are several classes numbered 4xx and 5xx, which are generally the classes taken by graduate students. The 4xx classes are intended primarily for senior-standing undergraduates, but somewhere about a third of the students in those classes are graduates, who will take the class for an extra credit hour (4 total) and are usually required to complete some extra work. There are generally a larger variety of topics to choose with 400-level classes, and I'm sure you'll find yourself taking at least a couple. 500-level classes generally cover very specific fields that you will want to take once you're a bit into your degree.
In general, classes that include undergraduates, that is, 400-level classes, will have sometimes upwards of 200 students. While some people find this to be a bother, it never really affected me all that much, and sitting around the front of the lectures made me feel as if the classes weren't that large. The usual format for these classes is to have about two lectures a week and, if the material requires it, one smaller discussion section with <20 people. On the other hand, the graduate (5xx) lectures are generally much smaller, and will very seldom have over 30 people. I can't comment on the classes at Cornell other than the fact that, on average, classes are supposed to be much smaller.
Because it's a large university, UIUC always has something to do. Usually, as a grad student, you'll be hanging out with other people from your department, but there's always the option of meeting people at bars and the like. I have gone out a few nights in Cornell and can say it's more or less the same thing, but with perhaps with fewer options. There are a number of bars in Champaign, and you're bound to find one that's full every day of the week. That being said, I have to say that after a few years in Champaign, I did grow a bit bored of going to the same places over and over. Of course, my friends in Cornell have said pretty much the same thing.
The standard of living in Champaign is quite nice, and over the years I've paid 300-500 or so a month for rent and utilities. Overall, I end up saving about 1000 from the stipend. I've heard living in Ithaca is a bit more expensive, and public transport isn't covered by the tuition as it is here. The scenery here is also very different from Cornell's, but that's just a matter of taste.
Champaign is roughly 2 hours from Chicago, and Ithaca is roughly 3.5 hours from NYC, so you have a bit city within driving distance from both places.
The types of people you see at UIUC and Cornell are pretty different. Here's what you'll see at each:
Undergrads:
These are probably the people you'll see the most of outside your lab and building. In Illinois, you're going to see a great majority of midwesterners who are from the suburbs of Chicago and have distinctive ways of dressing and acting. Because it's a state school, there's a good amount of party-animals and binge-drinking. In Cornell, being a private school, you'll see a bit of the opposite, with people mostly from upstate NY and NYC, so although there's still a certain amount of the undergrad party culture, it seems a bit more refined and high-class. In Illinois, you'll see a bunch of plaid shirts and Northface jackets, and in Cornell, people who are generally better dressed. Of course, in both places, there is a fairly large international population.
In your grad CS department:
At both, you'll have a strong majority of Indian and Asian students, most of which are international. Domestic students and women are a minority, but there are still quite a few.
Townies:
Seems to me as if they are generally avoided at both.
In the end, it really depends on your interests. Once you visit, you can decide whether you have a particularly good match with a professor and if you like the place. One thing that I can comment on, however, is that UIUC is a safe decision when it comes to grad school. It has a number of very good professors in almost all the subfields of CS, so if you come here and end up deciding you want to do research in an entirely different field than what you originally planned, you won't be in the wrong place. That's the advice I can give, after having changed fields myself.