It is debatable to claim which team is better. Juventus isn't in its best form anymore, the current good form in Serie A can't guarantee they will play well in Europa League as well. Sporting simply showed that they play well in Europa League, they proved it by eliminating Arsenal (the favorite team). If we compare Juventus and Arsenal, Arsenal is indeed better in league matches. So, since Sporting was able to defeat Arsenal, in theory they should be easier to beat Juventus.
Even though Arsenal have been removed from the league by Sporting, I do not believe they have a chance against Juventus. Juventus may not be good in their domestic league, Seria A, but they are excellent in the Europa League. They're more focused on winning the league because they don't have a better chance of winning Serie A this season.