It is difficult to make the previous match as a reference because even the closest one happened 6 years ago and was only like a friendly match with the situation of the players who actually changed quite a lot even though there are still some players who have felt it.
At the moment Arsenal is also a favorite because after all we must be aware that they are playing at home so their probability will certainly be seen as greater.
But on the other hand for now Madrid is still Madrid regardless of their performance in domestic competitions like what, they will still be one of the clubs that will be very dangerous in the Champions League so in the end even with Madrid's performance in the domestic competition being problematic, I will still look high for Madrid at this time and quite hope that they can get a lot of semi-finals.
Odds no doubts are in favour of Arsenal, one main reason for this might be because they are playing at home. In football, result of game may differ with odds and that we have seen so many times. To me both teams are equally capable of winning the first quarter final and it's difficult to predict the winner. Arsenal now out from EPL title contest race after they played a draw against Everton and they have full focus on UCL title now. Both teams have issues regarding player availability due to injuries and suspension, so let's just wait and see who wins the first quarter final.