These are all good and insightful comments.
Just to be complete, the island arrangement offers access around 360 degrees, or if one side is against a wall, 270 degrees. Around the walls is perfect in access and reach, but from one direction only.
Around the walls generally allows more space and opportunities for industries and passing sidings and a yard, so one can operate the trains realistically, making a run servicing industries.
Viewing angles are sharply limited with around the walls, you can only see the layout and trains from one vantage point, that is, the side that is not against the walls. I appreciate being able to see and watch trains from many angles as they do their thing.
Arround-the-walls requires at least one door and access into the layout, requiring a lift-out bridge or some such solution.
So perhaps the "ideal" arrangement is around the walls for wide diameter and smooth running, with a peninsula in the middle with (perhaps) a tighter diameter to make the curve-back. For instance O72 around the walls, but O54 in the peninsula. The two make a connection (various arrangements are possible). If both feature a loop, then two trains can loop/circle without intervention for train watching, or for letting the trains run while you are working on structures or details.
I tend to like a double-track main around the walls, so I can see trains running in both directions. Then as part of the peninsula or its connection to the around the walls, design at least one reverse connection to turn trains around. This also facilitates the out and back running scheme: from a yard out and around, turning direction, and return back to the yard.
