with the current conditions Man United are better than Barcelona, but I still think that Barca could beat them in the 1st leg and It will be enough to qualify, keep in mind that the style of play is different and Man utd can find it difficult against Barca if they take control of the match.
The level of play in the Premier League and the level of play in the Spanish League is very different this season and I see the style of play in the Premier League looks a little faster than the players in the Spanish League and that will certainly make the match more exciting to watch because each team will run their strategy during the game. Because Barcelona will also find it difficult if Manchester United players are able to control the ball more.