As someone who install's TV's for a living, I never like to see a TV above a fireplace, it's just to high up, plus keep in mind you have to get the wires to the TV from where your equipment is and working around a fireplace for that is always a huge challenge as well. I see in your case it will sit on the mantel, but still. Without seeing the whole room I would say look at your second location (between the windows) or try and find a place that is a little lower down. Will it work up there? yes, but I think you would be happier if it was elsewhere. A word of caution on between the windows though is I do not see power up the wall, and again you have to get the wires there, and on an outside wall its tough as you should not go through (like we can on an inside wall) due to vapour barrier. You may think about putting it on the wall to the left of the fireplace and window, on an articulating arm, allowing it to swing out when you want to watch from the couch, and fold up against the wall when your not watching, again all i said about power and wires apply there too.

I don't know if that helped or hurt but, a few words of advice from the TV guy perspective.

Joe