Overall, Juventus are the favorites, and what is a little doubtful about them is the inconsistency they often show. This is a home match for Sporting and usually the team that acts as the host will play all out, moreover they are only 1 goal behind in the first leg. I want to see what strategy Juventus will play in this important match.
It looks like the second leg matches for Juventus and Sporting will also be more interesting to watch because what you said might be true if the home team (Sporting) will try to outperform Juventus even though they are one goal behind on aggregate. And Juventus themselves must also have a pretty good strategy in this second leg because Juventus must know that the home team will play better to ensure they qualify for the next round.