Bookmakers give a slight preference to Inter:
To Qualify
AC Milan 2.14
Inter Milano 1.71
But in my opinion it will be a completely random game - literally randomness will decide everything, like who is lucky to score first or something like that. By the way, Milan are above Inter in the standings and Inter have a monstrous 11 defeats in Serie A... Milan seems to be the correct bet.
To be honest. Im a bit confusing seeing how bookies favoring intermilan rather than ac milan. i do know that if playing away or home won't give impact as the match will always be happening in san siro soon. Intermilan was just winning with lukaku has been showing his great performance again.
What you have said was also true if milan is above inter right now. I may also think of picking ac milan rather than intermilan.

I think that bookies may see the result from previous matches as the reference why bookies favoring intermilan compared with ac milan. As yo can see that if intermilan was quite dominating the duel in the previous matches.