Hey I am an undergrad at U of I and I have been here for about 3 years. It really depends on what kind of atmosphere you want to live in. Since you are a grad student you probably would want to be in a quieter area. I would recommend staying out of the campus town area, especially along Green Street because that is where a lot of bars are and it can get very loud any night. Waters Edge is by all the congestion of the main shopping area of Champaign-Urbana, I would probably avoid it. Ashton Woods is like the opposite, you'll be in the research park away from everyone else but still really close to campus. I have heard things about Town & Country, I think you should stay away from that.
Another thing to consider would be living near a bus line. The bus system in Champaign Urbana is really good, you can check out the bus lines at
Champaign-Urbana Mass Transit District. If you are a student you can ride any of the buses all you want for free which is great. Also a good rule of thumb is to try to stay south of University Ave in Champaign. North of that it can get pretty shady, that is where a lot of poverty is where lots of crimes happen.