There is another consideration in layout planning: how important is the scenery going to be? How much do you like to do it? What kind of scenery can you do best? Which season? Buildings (including bashing) or natural features (trees, streams, rock faces, etc)?
You can design and build a layout that basically is a narrow pathway for the track, extending not more than 3" from the most outside rail (whether main or spur). A skeleton of railroad built on regular benchwork, with large open gaps showing only benchwork support. Or you can cover the whole framework with a surface that is edge to edge scenery. The latter is the traditional and almost the only way layouts are designed and built. The one exception is the around the walls type: a narrow shelf holding only track, with no completely scenicked area across the room to the shelf on the other wall.
Lee's layout extension past his computer is a"skelton" design. Only big enough to hold the tracks on it, maybe some building flats across the back, and structures in the midst of the trackage. There will be no area of scenery running along it and made to blend into the main layout.
This is why I like your last layout plan, the along the wall. You are obviously going to build a very track-heavy layout (keeping all the track clean may be a problem) which will look fine without any pure scenery areas with no track in them. The railroad and its operations are the "scenery". Flats for factories along the back, loading areas and spurs for industries along the front -- those buildings are assumed to be just off the edge of the layout. And for any waterway or highway crossing cutting across the ROW you have a choice: do a whole scenicked area with all the expected elements, or model just the bridge and abutments with an open space -- no surface at all -- between them. Bridging literally a gap in the benchwork.
This is all radical, heretical ideas about layouts -- I've found very few people who like them. But I built my layout using these ideas because I wanted to spend most of my effort on track laying, maintaining it, and running trains. I also was tired of having to put scenery linking all the track lines together. They are all supposed to be different locations, not just the same as the line on the other side of the field/mountain/town! A layout with big open gaps you can see the support benchwork through may sound weird and and incomplete but you tend to ignore the gaps after a while because the scenicked areas draw in the eye completely. Then your gaze goes from a downtown urban area across a blank gap to different part of the layout of other track running through rolling forestlands and is somewhere else. Instead of trying to use continuous scenery to meld a city center with a completely undeveloped area in the space of a few feet or less, which I've yet to see anybody do plausibly without using different levels.
