I would have to say that the Cleansing Beam is far better.
It is movable, doesnt take so *** long to recharge, once you get it maxed out you can destroy a base incredibly quickly, it mops up most units, is not easy to dodge like the MAC blast, and to top it of, you arent screwed over economy wise early on if you use the cleansing beam whilst the MAC blast is not good at all for that (Mac Rush aside)
mah prophet is farin hiz lazor beamz