Hyatt Regency ORD is a "Regency" in the sense of incrdible atrium, glass elevators, health club, multiple restaurants and bars, including the revolving ones above the penthouse. They charge for parking even though it's an "airport" hotel.
Hyatt Rosemont is perhaps 1/10th the size, free parking, 1 restaurant, no entertainment, maybe 1-2 upgradeable suites (better chance at Regency). Food is fine; quiet property.
The nice thing about the Rengency is that it's connected via bridges to the Convention Center and all of the other hotels in the complex. So: if you're looking for a Hyatt and your modus operandi is to cacoon with room service, Internet connections and free parking, the ROSEMONT. If you like to experience everything from the sauna to the 360-degree nightclub and 24 hour room service then the Regency might be more to your liking.