The probability of Chelsea winning against Real Madrid in my opinion is much better in the first leg. Chelsea are playing at home and that should be the reason why they will be favorites to win over Real Madrid.
I don't think Real Madrid are far superior to Chelsea this season. Real Madrid's defense is not good, their midfield is not as good as a few seasons ago and they only rely on Benzema as a goalscorer. You will probably know that there will be a tight guard for Benzema in that match if he is really available, and that man-marking strategy will get Real Madrid in trouble.
Vinicius is the second highest scorer in La Liga and Real Madrid have a large pool of players who can score a goal. But in general, Chelsea in the EPL has similar statistics to Real in La Liga - Chelsea has +38 in goals, and Real has + 34. Real has a 69% percentage of games won, and Chelsea has 60% - here Real have a serious advantage, but it is more likely indicates a lower average level of teams compared to the EPL. In general, I think it will be a fight of approximately equal opponents.
For me Real Madrid are the underdogs in this match, I know they have world class players like modric, Karim benzema, Kross and Dani cavajar but some of this players have dacline over the years, players like Toni Kross and Marcello can't mark a good opponent for ninety minutes,but this Chelsea side are vibrant and strong they have good players all over the pitch that are young and reliable, Timo Werner, Pulusic, Kai Harvetz, zeyich and Mount this are all very good players that are all desperate to start against the Spanish giant, they still top it with experience like Ngolo Kante, Romelu Lukaku, ceaser azpilicueta and Thiago Silva so if you check things out you will see Chelsea have a better squad.