I would go with "whether". "If" is not incorrect but "whether" sounds better and is more elegant.

"If" expresses a condition that must be met for something to happen. For example: If the city was real, they would have been part in the discussion. On the other hand, whether represent a dichotomy, a situation where there are two options: "We don't know whether we are going to go or not." Although you would be understood in both cases, I would use whether in your sentence.