Personally it sounds like the older one uses his room less. He comes in later, only sleeps there, doesn't do his work there, so he doesn't really need a bigger bedroom. He's not there a lot. Plus the younger one has more toys and needs a play space for them.
I would put the older kid in the smaller room with a setup like the photo below and put the younger one in the bigger room. Put a couch or lounger in the room near the toys with the assumption that if the younger is not there, the older one can hang out there. If the younger one is there, the older one can hang out in the public spaces. Sounds like he's busy enough he doesn't hang around the house that much anyways.