What are you planning on doing? Where are you planning on going?

If you are just going to do the tourist sites in the downtown (central business district) area, then a car will be an expensive nuisance.

If your travels will take you beyond the city borders, then a car may be a necessity.

If you are going to be going to be visiting places outside the downtown area, but still within the city, a car may or may not be useful, depending on how widespread your travel will be (will you want to visit the NW side and the South Shore area all on the same day?), what kind of neighborhoods you want to visit (do you have a desire to explore the inner city?), and what time of day you will be traveling (beyond the lakefront and the area within walking distance of the Red and Blue lines there is minimal public transport in the wee hours).