If doing a CL, can design so returns aren't noticable (
see OM picture forum), something a little harder with PH unless using something like a
Tunze rock.
PH and CL can both also give noise. Might get a overall higher flow with PHs than a external pump for the same noise level if not running a separate fish room (do find my Dart though ok for being under the tank).
My tanks been up for a couple years with a CL and still no regrets.