If your trip is in August, I wouldn't think twice, it would be Chicago. During the summer, Chicago is full of outdoor concerts, cafes, movies, and sporting events. Not to mention the beautiful lakefront, lake front parks, and multiple beaches. To me, Chicago during the summer beats hot, sweltering NYC, which will be over run with tourists.

I'd suggest doing a little research in Chicago. There are some very famous museums, the Willis (Sears) Tower sky deck, the Hancock, famous architecture, and some excellent restaurants. Plus the people in Chicago are very friendly, especially in the restaurants and bars, but also at the Cubs game you'll want to attend in August. Although I have to admit, New Yorkers are much friendlier now than 10-15 years ago. More side trips from New York though. From Chicago, I would only recommend Milwaukee, which is very nice in the summer.

Having spent a lot of time in many large US cities, I would say these are probably the two greatest, and with a little homework you will have a great time in either one. I'd also recommend San Francisco and Washington DC.