Nobody really "knows" what is going to happen, there is no reason why we should argue about any of this, we are just speculating, just like crypto prices. We can guess that Real Madrid will win because they are the better team and that should be clear enough to make them favourites and have them be wagered a lot more than Arsenal or even a tie. But that doesn't guarantee their win, so people who say Real Madrid will not win are also right as well, because there is a chance that might be true too.
So do not argue with each other on this, you could see this being a very true deal or a very wrong one, you have no idea which one will happen, so no reason to have heated arguments over it. Just bet whatever you personally feel like it.
But discussions are what this topic is made for. The odds are 2.38 for Arsenal and 3.05 for Real Madrid. I thought the odds are more equal than that as the bookies favor an Arsenal win more noticeably than expected. But this has to do with how both teams played their games as of late. I think for Real Madrid fans and those who like to bet, they are not getting a bad deal with 3.05. Tripling their money if they think Real Madrid is the most likely event, not the worst deal.