I grew up in the south suburbs of Chicago, which has mostly been Sox territory, but I grew up rooting for both teams (blasphemy, I know). My father was ambivalent about sports, so I really didn't grow up with a family tradition. My grandmother was a big Cubs fan and I'd go to a few games with her. But most of my buddies growing up were for the Sox so I grew up never really picking a side. I went to games for both and still do. It's nice to have a team to root for in both leagues. Call me fair weather, I don't really care.
There's no question there are more Cub fans than Sox. I would say it's around 70/30 Cubs in Chicago. At the risk of oversimplifying it Sox Nation is roughly Madison St south extending to the south suburbs with Bridgeport (where Sox park is) at its core. Sox Nation is a few square miles. Really, the Sox are like a small market team trapped in a big city. Contrast this with Red Sox Nation, which goes from the tip of Maine, includes all of New England down to CT, near Yankees territory. The Cardinals own the heartland of country, not just the St Louis metro area. NY is so big, there's plenty of room for two teams. The Cubs have the Northside and much of the remaining suburbs and much beyond judging by the fans that come out on the road.
But let's not forget that the Cubs used to play to empty seats, too. It's only since the Wrigleyville area has undergone gentrification that it's been cool to be at Wrigley. In the early 80s most derided Wrigley as an old banjo box and it was certainly no neighborhood you wanted to hang around at after a game, day or night. So, obviously, they get a lot of the tourist crowd that will go see a game and have something to do afterwards. Let's relive Lee Elia's famous profanity filled tirade against the fans in an
'83 press conference to recall where this team once was. Look back at some of the games before the Cubs finally had a good team in '84 and you see nobody on the roofs because few cared.
The Sox are in Bridgeport and while it's not as bad a neighborhood as many seem to think it is, it just doesn't have the allure of Wrigleyville in this era. While I don't think we'll see much of a shift in that 70/30 number next year, it could happen over the next decade or so. The South Loop has been undergoing it's own gentrification, old projects are getting torn down and who knows, it could spread to the Bridgeport area and it could become "cool". Even if that doesn't happen, you'll see a slow shift to the Sox if the Wizard of Ozzie stays and this team starts making playoffs more regularly.