Both teams have been very bad this season and I don't even know who is worse but when it comes to the Europa League, they have been doing very good especially Manchester United that have remained unbeaten. I also agree with you that Manchester United are the favorite to beat Tottenham whenever they meet and I have the feeling that Manchester United will win the Europa League as their only hope of playing Champions League next season.
The same way Manchester United is relying in the Europa league trophy to get a qualification to the champions league is the same way Tottenham Hotspur is also depending on it. So therefore, I can’t rate any of them over the other because of this and they will both give a very tough time when they meet each other in the finals. Manchester United may have been on an unbeaten run in the Europa League but that’s not a guarantee for a sure ticket to prevail again Tottenham Hotspur.
I wouldn't underestimate Athletic Bilbao and think they are even a better team than Manchester United. They are actually 4th in the league and in a CL spot while ManU is in the bottom third of the table and having one of the worst seasons in the club's history. If ManU had played Bilbao a few years ago they would have been the favorites but today I think Bilbao is the favorite.
Manchester United this season have been so poor that their game is not even encouraging to watch. Despite playing well in the Europa League so far this season, they’re just not the team anyone is trusting on an 100% that they’ll win the Europa League trophy. Atletico Bilbao will be a big threat to them because they’re better than them and playing better even from the league side.