[Jack]

I'd go with improved cylinder.
If I was super rich and could own 2 O/U's for upland birds like grouse, I'd have one choked cylinder/improved cylinder. I'd use that gun as long as the leaves were on the trees. When the leaves fell, I'd go to an improved cylinder/modified set up.
Another factor is whether you hunt over a pointing dog or not. If you are hunting with a pointing dog, the bird often is within a few feet of you when it flushes- and you can't have a choke that's too open. Cylinder would be perfect.
If you're hunting without a dog, or with a flushing dog, the birds will be a bit farther away when they flush. Under those conditions, improved cylinder or modified would be best, IMO.