On paper, Leverkusen is still more reliable than AC Milan, of course I do not ignore the fact that they have just defeated Inter Milan who are the defending champions in Serie A.
Leverkusen in this season is a different team, they are no longer unbeaten with a first loss in Bundesliga and AC Milan just woke up to beat Inter Milan. The match between Leverkusen and AC Milan will be balancing and I don't underestimate AC Milan in Champions League.
For many decades, AC Milan shown that they often play well in Champions League even they did not have good form in Serie A, so people call it as AC Milan's Champions League DNA that does not exist in Bayer Leverkusen.
In terms of experience and mentality, Leverkusen is clearly far behind AC Milan, but the current situation and conditions are certainly different. In my opinion, this match tends to end in a draw, even though the odds displayed place Leverkusen as the favorite.
Leverkusen played in Champions League some seasons and had one failed Champions League final under Real Madrid with a magic volley of Zidane. Generally, they have less experience in Champions League than AC Milan in history. With current players in two clubs, AC Milan have more players who already experienced Champions League in previous seasons, and this can be an important factor for difference between two clubs in their direct match.