We still don't know exactly how much the odds, when look at stake, Barcelona have 1.75 while MU have 4.30 and the chance of draw is 4.30. Maybe this still temporary result until next month begins. But, we know in home, Barcelona is a club which is not easy to beat, MU performance is far away from expected, especially Antony which have a gap different like he played on Ajax
We probably won't see a big difference in the odds when the game starts playing, like you said Barcelona playing at home is a team that is not easy to beat and this gives them an advantage in this first leg, if United can make this first leg match draw then the opportunity will be theirs, but it will be a tough job in the match later
I think this match will be quite fierce, considering that both teams performed quite well in their respective leagues. Moreover, Barcelona's winning trend is still slightly above Manchester United, but it's not certain later whether Erik ten Hag will try a new strategy against Barcelona later. Even so, at least Barcelona must win the first leg at home, so that their steps will be easier if they later play the second leg away at Old Trafford. So I prefer small odds for Barcelona.