Like Wid said, whatever's closer to ear level might in theory be a bit better. However if there's no significant difference I would just go with whichever looks better or is easier to install. Your room calibration will take care of any minor differences between the positioning (Audyssey, MCACC etc...).
I am assuming you're not worried about a 2nd row of seating. If that were the case i might consider going above to maintain better dialogue intelligibility for the 2nd row, but again that might be very small differences.
Look forward to seeing some pics of your install.
Cheers,
