1) I love Chicago, but Rio is the best choice overall. Probably better for most people in Chicago (I speak from having lived through the ramp-up to the Beijing Olympics these past few years), although some of them may not feel that way right now. Certainly better for the whole spirit of the Games.
The US has had a lot of Olympics; no country in South America has had any. I think that these events feel more special, and get a better all-out push from the host country, when they represent some kind of inclusive "first ever" achievement. Japan marked its post-WW II recovery as before and after its 1964 Olympic games. South Korea in 1988 and China in 2008 used their host role, in different ways, as big national milestones. Athens in 2004, too, had some kind of closing-the-circle fulfillment in bringing the games back to their original home. Whereas for Los Angeles in 1984, Atlanta in 1996, and I assume London three years from now, the Olympics are mainly big logistics challenges to be coped with and endured.
2) Obama was in a no-win situation about his personal lobbying. The other candidate chiefs of state had also made personal pitches. If he hadn't made the trip and Chicago lost, you can imagine today's right-wing theme being: he didn't care enough about his country even to try. (Probably because it's not really his country. Now, if Kenya had been a finalist...) Since he did make the trip, the theme is: what a loser.