No offense to John, but I'm not personally a fan of ceiling layouts. Unless you have low ceilings, a wide room, or an elevated seating area, the viewing angle means that you won't be able to see much of the trains, or any scenery that you might be inclined to add. I also haven't seen a ceiling layout that provides any kind of varied operation beyond parking one train and running another, and running in circles will get boring fast. Access is difficult, even if you just want to change out the consist. Like I said, I'm just not a fan.
I live in the Land of No Basements, and sacrificed one of my 3 bedrooms to get the 11'5" x 18' that I have. Because I needed a home office AND a guest room for long-term visitors, I paid a carpenter to build a Murphy bed AND a Murphy desk in the 2nd bedroom. So now that room can be an office or a bedroom, just not both at the same time! If you have an extra bedroom, maybe my Murphy solution will work for you!
I would avoid the living room, so that leaves the den. I'm not sure about your room dimensions, but I endorse the idea of a corner layout. If you go this route, I suggest a long point-to-point built on shelves along the walls, with loops at each end. I would use the absolute smallest radius that you can tolerate, probably O36, or even O31.
What kind of trains do you have? For many years I bought only "scale," including about 20 large mainline steam locos. Within the last decade I've come to the unfortunate realization that, as nice as they are, they just aren't house-friendly. Disclaimer: I'm not a "modeler;" I've never painted or weathered anything! So even if I had more room, I'm not sure that the resulting layout would be more realistic. Once I accepted the necessity of sharp curves, traditional-sized trains and structures, I've been able to come up with plenty of interesting layout designs.
I empathize with you and I know this process can entail difficult choices. Good luck in your decision process!