It's Chicago -- assuming you don't have protected parking at night and Chicago winters are still brutal, I'd drive my hard top way out of the city to find a storage room to keep that top. Come winter time again, you may be sad you got rid of it for a couple hundy.
I know when I lived in NYC, storage spots in the city were expensive but $20 in gas got me to a spot where monthly rates were more like $30/month. Find something similar in Chicagoland, and then you can figure out what kind of soft top to get. Keep in mind that your hard top is going to be harder and harder to find each season on the used market (read: more and more expensive). Therefore, I would concentrate more on where to store it that doesn't cost you too much.