Having been in the club a number of times the past couple years, I have only seen it what I would call packed once and that was a day that there were IROPs galore. It is very often a peaceful club (by SC standards nowadays) and not a bad place to get away from the masses at ORD. Maybe more a function of the airport and how tired and old and crammed the ORD airport is but the ORD SC is one of my favorite in the DL system purely because it actually does feel like a "getaway" compared to any other one in the US.

Personally on the layover if I were the OP, I would probably find a Chicago style jumbo hot dog or maybe some deep dish pizza somewhere in the airport or take the L a few stops if you really want to get away. Then spend some time in the Skyclub which normally isn't too busy in the evenings before the later flights from my experience. Have a drink, surf the web, enjoy some FT and have a snack.

Had it been MDW, you could have easily made a 5 hr layover possible for a lot of time in the city as MDW is about 25 mins from downtown by way of the CTA L train. Plus no SC at MDW making it an easier decision.