You're neglecting the important detail in that Colossus-Slayer is the most abjectly boring option to pick from the available options. Sorry... it really is though.
In tabletop, I've found that the instances where you're taking your turn and you wish to ask "has that one been hurt yet", and the answer is no, comes up more frequently than individual impressions feel like, and conversely, the situations where you can get that extra free shot in because there are at least two enemies close to each other, happens fairly regularly - on account of melee creatures tending to try to attack the same targets, if nothing else.
This isn't table top though. For most of the encounters in EA and because of how enemies seem to behave colossus slayer is simply better. Now whether this will be true on release is certainly a contention, but until then colossus slayer is the better pick.
