I'm away from my club book right now, so I can't actually check the rules, but from what I remember, you must enter the CH class. Once the dog is eligible for either the Ch or Working class, they cannot go into the open class. Open is for dogs over 24 months with no working (SchH/IPO) title and no Champion title. I think the only time you would have a choice is when the dog is both a Ch and a SchH titled dog (like Chris's dog). I would say you need to go into champion class.
Besides, I don't think it will make any difference to the judge. Your dog will be critiqued on his conformation as it relates to the standard. You will (almost certainly) have less competition in the Ch class than in Open, for sure. But, if he wins the Champion class, he will go against the winner of the Open class (and working, veteran and sieger classes) for Select Adult Male.
Enter the Champion class. It really is the best choice (especially if you have no other choice
).