Which currently impressive infielder would you rather pick up for the push toward the postseason?
At this point in the season you shouldn't need/want either one. However, Ackley has more position flexibility...but Arenado has more power and has the better home environment.
...but in a week with the expanded rosters, some sleeper will emerge as a must add. Especially for you H2H (why bother play FB) where the only thing that matters in FB is 2 weeks in the playoffs.