I don't think with Arsenal and Barcelona only are the team have favorite for Europe League winner because still have Juventus back to consistency performance at last six matches, right now Europe League tittle race most competitive for winning the second competition level on European. Honestly, Arsenal have better quite appearance in this season at domestic league and European League but I doubt can consistency because next round of European League begin on February next years.
Many predicting the winner of European League between Barcelona and Arsenal but the problem right now Barcelona have against with United on play off round.
As long as Barcelona haven't made it past the playoffs, I will never favor them to be in the Europa League final. Manchester United is not to be underestimated, so currently both teams have the same chance to qualify for the last 16. It's true that Barcelona is expected to win the game against Manchester United, but I won't expect too much before the match starts and see how the team is prepared for that match.