I would definitely recommend Galloping Ghost. I mean, if you're in the Chicago area or passing through and enjoy classic arcade games, do yourself a favor and check it out. In my opinion, it is a sight to behold.
Yes, there are A LOT of machines crammed in there, so it can be pretty cramped. From my experience, the best time to go is during a weekday afternoon, while school is in session. It can be very dead. Late afternoon and evening, it tends to get pretty crowded. Some aisles are difficult to navigate because of close quarters and too many people. A weekday afternoon during the summer probably wouldn't be too bad. But I would probably avoid going on the weekend. The amount of people might ruin the experience of just being able to walk up and pretty much play whatever you want.
Nothing against the Underground Retrocade. It's a cool classic gaming arcade, as well. It's roomier, for sure. And they have more pinball machines. I haven't been there for a while, but on my last visit, they had close to 10 pinball machines. Galloping Ghost only had 3 or 4 on my last visit in May. They're both fun places well worth a visit. But if I'm going to recommend one, it would be Galloping Ghost. The amount of games just makes it so impressive. Check out their website for a list of games.