There's a secondary benefit of going around-the-walls vs. the island-in-the-middle. The physics of running the train. With the island, you see the entire layout (assuming no view block in the middle). It makes the layout look smaller. Additionally, you see equipment rounding the curve from the outside which exaggerates the "coupling gap", plus you see the rails bowing out underneath the cars.
With an around-the-walls, you can't see the entire layout (unless like some moms you also have eyes in the back of your head). This forces you to follow the train's motion. You also see the equipment from the inside of the curve which reduces the coupler gap. With longer cars, you don't see the rails bowing out from underneath which hides the sharpness of the curves.
Speaking of sharpness, the island will require sharper curves. In a 12'x12' room, you're pretty much limited to an 8'x8' island (or 8'x10' peninsula) for 2-foot aisles. This limits your curves to 44" radius (O-88) and 48" of straight between them maximum fir 96" of straight total. Using 2-foot shelves on around-the-walls, using that same 44" radius (O-88), you can have twice as much straight track (192") using a square layout with a lift-out corner. In the example below (42" radius/O-84 with 4" extension of straights, plus a beveled corner for a lift-out), the straight trackage totals 175" (some loss of straight because of the bevel). It allows for placement of a passing/run-around siding.