Most important in design is viewer comfort so thing about how you will be spending your time viewing and where you will be viewing from (location of seating).
Most of our installaions have gone over the top of the mantle. Given the height I would recomend at the least a wallmount bracket that tilts to adjust viewing angle (the bottom of the monitor is at 60 so would put the center at least 18 inches above that) but would recommend an articulating bracket so the monitor could be set to any viewing angle. The side by side works as well and brings the monintor closer to the floor. Once again you need to ask yourself what will be my primary viewing position/location.

The arrangement of tv/fireplace wall is a classic dilemma! For comfortable watching the tv height should be between 40-50" depending on the viewing distance. The centerline of tv should be slightly above the eye height so the person's head is slightly tilted up. Between the two options I prefer the side by side arrangement. The tv height may be lower than ideal, but having the tv above the mantel will require the watcher's head to tilt up which will not be comfortable for sustained viewing.

Id put the fireplace down, TV up and small shelves (3-5) on both sides.

I would only go stacked if you lower the fireplace to be flush with the floor, eliminating the seat. This is a more modern detail and is consistent with you open-plan ranch. Then the TV could go above and not be too ridiculously high. Otherwise I would suggest the side-by-side, as I have this arrangement in my house and find it much more comfortable to watch a 'low' TV than craning my neck upward to watch one that sits too high.

That said, it's not going to be easy to make this thing look right no matter which you choose. If the TV is wider than the FP and you stack it, it will look unstable with the wider TV 'balancing' on the skinnier FP. If they are different heights and you go side-by-side, you run the risk of having a strange composition as well.

Have you considered eliminating the FP entirely? They're not much good anyway as far as heating is concerned, generally look pretty 'fake', and absorb cost that you could use for a well-crafted group of cabinets built-ins that properly house you TV, cable box, etc. If you're going for heat, there are some great Scandinavian wood stoves that are modern, efficient, and quite beautiful - one of these could be located anywhere in your great room, somewhere near the center?

Just some ideas to help you keep options open. Good luck!

Kyle

In this situation a stacked TV / fireplace looks more proportional.

I don't know if you have a gas or wood burning fireplace, but if you have a wood burning one the TV will melt!!

I like the stacked method. If you have a wood burning stove, you will need a place for the irons and it really is a more modern concept.

I like the second design. Gives the wall greater importance and nicer lines.

Check out this photo in my latest Laguna Beach project that has that layout.

Beautiful room. I like the TV/fireplace combo. Very sharp!