You could visualize the antenna pattern of the AP-135 as a cone, but one with a very narrow angle and wide base.
If you ceiling mount the AP, this cone is basically on its side, and a significant amount of energy radiates towards the ceiling and floor. For many indoor deployments that may not be an issue, since energy is reflected "all over the place" anyway, but it may result in client devices connecting to APs one or several floor down/up, which may not be desired. Also, the horizontal range may be reduced a bit.
An AP-134 with the stick antennas I mentioned actually will have better horizontal range than the AP-135 ceiling mounted.
/Onno