I don't think the Celts will get swept.
I do think the Hawks are solidly a better team, at least in the playoffs.
Stats wise, the two teams won the same number of games. Their point differential was about the same. Both teams were about .500 on the road, and significantly better at home.
The Hawks were a below average offensive team (tied 17th in Off Eff). The Celts above average (12th).
The Hawks were the 2nd ranked defense by a significant margin. The Celts were 6th.
Both teams are crappy on the defensive boards.
The Hawks are a top 10 team in shooting. The Celts bottom 10.
This last may end up being the true difference maker in the series.
Overall, the Hawks are the better and more experienced team. Most notably they have a major advantage at the big positions, though they lack depth there.
Based on all of this, I think it'll be a hard fought series, but the Celts will have to play exceptionally well to avoid losing in 5.
Generally, when a matchup is relatively close but one team has a clear edge, and that same team has homecourt advantage, the series tends to end in 5.