I'm favouring Juventus for advancing to the semi finals. I'm aware of how successful Sporting are in this tournament.
However Arsenal and Juventus are not the same type of teams. Each of them has a special kind of game plan for themselves. Sporting's game somehow worked well in the rematch and it led them to even eliminate Arsenal which were the biggest favourite team for the Europa League title. But Juventus' style can be really boring and maybe problematic at the same time. If Juventus outweigh them they can get past them narrowly on aggregate.
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 were just unlucky in the match against Sporting and Mikel Arteta's squad doesn't seem too disappointed with the result of the match because they still have a definite target of winning the domestic league title this season. And maybe it's true that as you say juventus is not going to win easily against Sporting because I think Sporting alone can hold back so many chances from Arsenal and they were able to get rid of Arsenal in the previous round. But victory in the first leg will be a serious bet for Juventus to surpass Sporting on aggregate points later.