I think it depends on how much art you plan to hang: how many pieces, the variety of sizes, etc.

If you use the sofa wall, the gallery can start over the sofa and grow organically if you are actively collecting pieces.

IMO use the FP wal for the TV hung on a n articulating arm at the proper height and use the 2 low cabinets on either side for storage and on the one side with no TV a nice piece of art then above the sofa a nic elarge piece of art. I do noy like gallery walls usually and honestly do not think the trem is the right one. I have hug gallery shows for many years and the walls with art are done to make the best of the space and show off the art . Most residentail gallery walls are just too busy or loaded with family pic which IMO belong not in piblic areas .

Why are you making an assumption the OP has bad taste in art or will only hang family photos?

I love gallery walls, so I think either space would work - depending on sizes, as palimpsest mentioned. That said, the wall over the couch is a nice big wall to work with.

If you're looking to do a gallery wall, we might suggest doing it behind the sofa. Here are a few ideas for you that might help you more:

Dupont High Rise: Living Room · More Info

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Gorgeous!!!

Good taste in art or bad, you still have to dust each frame, and will be constantly straightening them. You can get adhesive poster putty on Amazon to hold the frames in place. Though I think art on either side of the fireplace and a large piece over the sofa would ultimately look better.