If you hunt the relevant threads here you will see that actually the Concorde Room has quite a number of food options not available in Flounge. In particular there are 3 Concorde Room menus, for breakfast, rest of the day, and pre flight supper, which have a number of items not available in Flounge. Particularly the last menu.
But if you did upgrade to First, there is a case to wander into Flounge first, to see what is available, then go into Concorde Room. That way you can order anything from Flounge as well as the CCR specials.
There are additional wine options in CCR not available in Flounge. I've never known CCR to run out of the Laurent-Perrier either.