Great space, Ken. I think you might miss not having a good size yard to store trains on and would vote for the initial design. Perhaps just an O and S layout rather than three just to keep some open space - maybe an old-fashioned pinball game, too ! I am also a fan of watching multiple trains run and interact on double mains and, with command features like Legacy and the ability to set constant speeds, you can keep multiple trains running for a very long time w/o any concern.
I know you said you had a loft for storage, but you might also want to consider some Glen Snyder wall shelving for at least some storage and display of engines and rolling stock.