I recommend running it around the walls. That would enable you to use O-72 or larger curves and run the largest O-gauge locomotives. In that case, you might consider cutting down each section to 30-inch or 36-inch width for easier access. I have two completed island-type layouts (12'-by-8' with O-72 and 10'-by-5' with O-54) but now think around-the-walls is a better choice because it allows for longer loops, larger-diameter curves and easier access.
Great points....thank you!