I've visited Budapest several times, but not stayed at either hotel. I normally stay at the Marriott which is right next to the IC. I would recommend the IC purely based on location. The IC is right on the Danube (which is the center of activity and nightlife in Budapest) and less than a 5 minute stroll from the Chain Bridge. The Danube at night is a beautiful sight.

McGallery is located about 15 minutes away by subway+a short walk.

Tripadvisor rates the IC significantly higher than the McGallery as well.