For an inexperienced flier, more time is better than less. Like someone else posted already: if you're early, have a drink or a bite to eat. With multiple flights (I am sure) available from DFW-ORD, taking an earlier flight also provides a cushion if the flight is cancelled, as another possible connecting flight will be available in the event a rebooking is needed.

I am an experienced flier and when I connect - especially through a major airport like ORD - I book the extra time. Of course, as an experienced flier with the right credentials, I also have a lounge available for killing time.