Hm, let's see ...
Cerberus vs Collectors
Atlas vs Pretorians: Atlas would lose but it is grind fest, a slow battle and this is exactly what Cerberus wants. They would pin down the most dangerous collector unit with minimal cost (i.e. that is the very role of atlas: it is huge bullet sponge).
Scions vs Phantoms: again I would expect collectors to win this matchup but it will take a long time. Scions will be busy meleeing elusive phantoms which means that they won't be using their handcannon or carpet bombs.
Dragoons vs husk: no match, dragoons win easily.
Dragoons vs abombs: well, even if abombs win technically, it is a draw.
That leaves the remaining cerberus units vs the remaining collector ones: trooper, centurion, engineer + nemesis vs. trooper, captains, + bees.
Don't tell me that a turret farm supported by nemesis+centurions cannot mow down any number of collector troopers/captains/bees. It is not a contest.
So even if pretorians, scions win, they will have to face the same turret farm + nemesis/centurion (or even larger one, assuming that engies can plant more) without any support.
I would put my money on cerberus. While collectos have the most dangerous boss/subboss units, cerberus is more diversified. Collector's strenght is focused in those two type of units, pin them down, allow engies to set up the turrets, support them with smokescreens + snipers and Cerberus wins easily.