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.
Tottenham are slightly worse than Manchester United in the Premier League, but in the Europa League both teams have shown different performances, which has made them reach the semifinals. Tottenham also have the same hopes as Manchester United, they are targeting the Europa League trophy to get a ticket to the Champions League next season.
Tottenham journey to reach the Europa League final is slightly easier, because they face Bodo/Glimt who are now considered dark horses. While Manchester United journey in my opinion is very difficult because they face Atlethic Bilbao, a team with almost equal strength with them. In theory, Manchester United is considered to have a slightly greater chance of reaching the final.
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.