Neither position is ideal, obviously. The mantel position will arguably provide improved sound quality, but will be a much more restricted front soundstage. The corner positions, conversely, would provide a very wide soundstage (likely too wide, given the size of the speakers, so the pans wouldn't be as smooth left to right), but because of reflections and corner-loading of the speaker the sound would be more compromised.
So, of those two positions, I would vote for the mantel. However, if you can wall mount them in the corner, is there anything specific preventing you from mounting them about 2/3 of the way between the TV and the pictures on either side (i.e., closer to the pictures), which will help widen the front soundstage a little bit? If cables are the problem, you could always cover them with a plastic-type strip and paint the strip the same color as the wall.