Re: San Diego or Chicago?
Chicago is very tourist friendly.
Lots of museums are within easy walking distance of downtown and are some of the best in the world.
Check out the Shed Aquarium, the Field Museum (history), the Art Institute of Chicago.
If you'll be here September 3-5, you'll be able to catch Lollapalooza.
I would recommend going for just one of the three days to get a taste, otherwise it can be overwhelming.