I'd say most people I know use one or the other exclusively. I think Uber actually tends to be cheaper at busier times than Lyft if you tip when you use Lyft. The average Uber vehicle seems newer than Lyft vehicle as well.
Either way, I don't think Lyft is at much of a competitive disadvantage in Chicago, at least in the 20-something crowd. The older individuals I know are much more likely to use Uber exclusively, and use the features that are not present in Lyft, like calling a standard taxi, or Uber Black.
It's all anecdotal of course, but I personally wouldn't worry too much about Lyft. I think the fact that they appear to allow a broader range of vehicles and drivers is a competitive advantage.