Legal Question in Real Estate Law in Illinois

Technically you could sue both, but the first step shouldn't be going directly to court. Assuming this occurred recently, you should talk to your neighbor if you have a decent relationship with them. When they hired the painters, the painters should have provided your neighbors with liability insurance certificates. What normally happens in a case like this is you will get estimates for repair of the wall and have your neighbor put it through as a liability claim on their HO policy, and their insurance company will then go after the painters' insurance. It is best to document everything for your (and your neighbors') protection, so I would get an attorney involved to help coordinate things, send appropriate notices, etc.