With no bag to pick up, I would go for the 5:15. Assuming your plane lands on time, you are landing after the busy period at O'hare and you can make it to the terminal 3 in 45 minutes. Many times the lines for U.S. citizens are longer than the non U.S. but you can always go to the non-U.S. if it looks quicker. If your plane is on time you will make it