Both Inter Milan and AC Milan seem equally likely to win. The performances of these two teams have been erratic in Serie A this season as well. So there is no way to say for sure which team will qualify for the finals.
I second this, the game is fairly even at 50-50 and i absolutely don't think there is any favorite in this game. AC Milan and Inter Milan are quite erratic and unpredictable this season, they could lose when you expect them to win easily, and win when you think they are definitely going to lose, it has been an up and down season for the both clubs, and they are really fortunate and lucky to be in the SF of the CL. This match is going to be decided by fine margins, i prolly even think it will go into extra time, that's how close i think it'd be, but if i must pick a side, i'd say AC Milan would progress to the final, just because i think they have a game changer in Rafel Leao and a great goalscorer in Olivier Giroud, they would both help AC Milan to just edge Inter Milan.
I actually agree with @Coyster because, at this stage of the Champions League competition, I don't think there is going to be any favorite. Unless it is Real Madrid we are talking about. A lot of people might think that Inter is not performing well in the Italian League. That's why they might not be able to perform in the Champions League as well. But I will say that Inter will perform well in the Champions League because they are not performing well in the Italian League.
They will privatize the Champions League a lot more because the performance of them is not good in the Italian League. And that is going to make this match a lot more interesting and competitive. A classic Milan Derby. And let's also not forget that it is going to be a two leg match.