Sporting proved something great against Arsenal but I think they were just lucky at the penalty stage. Meanwhile against Juventus, I think Sporting just won't be the favourites.
You can say it was a luck factor only, but they can't win the penalty shootout if they have bad shooters (bad players).So, it simply showed that Sporting has a good squad and they used a good tactic. That's a logical reason why they can eliminate Arsenal in the previous round. If Sporting has bad players, there will be no penalty shootout, Arsenal may win the match in normal time.
I may have underestimated Sporting at this stage as I tend to believe Juventus are the better team.
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.