IG-88 >>>>> HK-47 in coolness.
I like IG-88 more, but he got beat by Dash Rendar (at least in game).
HK-47 helped Reven defeat force sensitive enemies (at least in game).
Judging both from an in-game perspective, HK-47 seems like the more accomplished droid.
IG-88 post SotE became the Death Star or some crap like that, so I think it becomes an unfair comparison at that point.