Hokie I've been thinking about big-picture considerations related to your train room. A very important question that only you can answer is, will you be content with one or two trains running around ovals?
Do your existing locos require O72? Do you prefer the wide radius for the sake of appearance? Using O72 curves on a 10 x 13 layout leaves only about 6' for your straightaways. It will be difficult to camouflage the fact that it's an oval layout, if that matters to you. I would be really tempted to banish the computer desk and train storage to another room. Maybe even re-hang the 30" door at lower right so that it doesn't open into the room. In other words, use the entire room if possible, instead of just a 10 x 13 island.
Everyone runs their trains differently. Case history: My own layout area is about the same size as yours, and I had originally planned to do an "open doughnut" like you. But I was afraid that after it was built I would get bored with it, and for all the money spent, it wouldn't see much use. I really wanted operational variety. The more I messed with my plans, the more I realized that 11 x 18 is plenty for "traditional" O gauge, but with hi-rail equipment and O72 curves, you're pretty much limited to a basic oval. It was a tough decision, but I'm probably dropping back to O42 or even O36 curves and I've drawn several exciting plans that fit the space.
Before you buy or build ANY benchwork, I encourage you to get a copy of John Armstrong's "Track Planning for Realistic Operation." Look especially at chapter 6, Pike Locations and Shapes. Mr. Armstrong uses a technique called "squares" to determine what track arrangements will fit in a given area. He also has a figure titled "An alphabet of walk-in pike shapes" that illustrate the more convoluted layouts that can be built if you're willing to step back to sharper curves.
There is no one "right answer." Most people are content just to see their trains moving closer to eye level, which a basic oval allows you to do. But no matter your preference, it's much easier to change the design BEFORE you order the wood for benchwork! My $.02.
