Hey supmello,
Congratulations on your acceptances! Times New Higher places UIUC at 36th globally and UCSD at 41st, so you could say they're pretty close in that sense. In my field, both are considered pretty good places, but I am more familiar with work done at UCSD.
I am not familiar with AI/Systems, but you could take a look at the faculty in each school and see if their research or classes are exciting to you.
I know people who studied at UCSD and they loved the city and atmosphere. I also know people who studied at UIUC, and while it may not be as exciting it doesn't have to be all bad, especially since you'll only be there for a shorter period as a MS student.
All things being equal there's nothing wrong with picking a university based on good living conditions, especially if the programs are pretty even.