No personal experience with either setups, but if I were in your situation I would opt for the sofit placement angled down towards the seats. I think the soundfield would be more seamless and uniform between the LCR speakers that way.
Placing them beside the fireplace might make the centre speaker stand out too much in relation to the LR because it'd be sitting so much higher. My other concern would be that the LR would be recessed / blocked by the sides of the fireplace (in the picture it looks like the fireplace protrudes out from the wall).
The only argument I can think of for side fireplace placement would be if you use them primarily for stereo / music listening, but I'd still be concerned about the fireplace sides blocking them.
