Arsenal's team was more stable and stronger than Juventus's. However, they lost against Sporting's match tactics. I wouldn't say that Sporting performed too well against Arsenal. Arsenal performed well in both matches but failed to qualify for the next round. There is no reason to think that Juventus will easily win against Sporting. The first leg is Juventus' home match. If Juventus don't win at home, I don't think they will qualify for the semi-finals. Juventus must win the first leg.
Arsenal is better than Juventus and yet they still lost but that doesn't mean that Arsenal played the way they normally play. Look at how they play during the league games and look at how they played against Lisbon, the two teams you will see are very different.
While they were amazing at the league games, they were not that great against Lisbon and that is why I do not think that there is a reason why there should be any doubt in mind about Juventus winning. Basically all I am saying is that Arsenal at the league is better than Juventus but the Arsenal at europa cup was worse than Juventus and that type of difference is why Juventus will win.