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.
Today performance will tell which of them is the worst. There is somethings Manchester United are bigger than. For instance, to win Bournemouth this season shouldn't be a problem to them, but they are already losing today as we speak. Tottenham have a game soon against Manchester City and I will be okay if they lose to Manchester City because City are better than them but if it was Tottenham against Bournemouth they will try to win the match. In conclusion both teams are completely terrible this season but from the look of things Manchester United is the weakest. Don't be deceived by their undefeated streak in the Europa League.