Real Madrid was the favorite from the beginning of the Champions League competition, and for them to reach this stage and keep performing as expected, they are still the favorites and can surely win the title.
Dortmund is not in good form, they ended the season below the top four spot which means if they didn't win the Champions League, they will be playing in the Europa League next season, that is why they will try heir possible best to win Real Madrid, which is possible but almost impossible because because Madrid is a better team and in good form than Dortmund, unless if Dortmund is very lucky just as they have been lucky all these time.
I would like to slightly correct your statement. Even though Dortmund failed to win the UCL trophy, they will still play in the UCL league next season because the Bundesliga has added UCL league slots. Last season there were 4 teams that qualified for the UCL League, but this season there are 5 teams. Dortmund finally finished in 5th place so they still have a place to play in the UCL League next season even though they lost to Real Madrid in the final match.
This final match is very interesting because it is the last final for Toni Kroos and Marco Reus. Because both of them are great players in the German national team, in the coming years we will no longer be able to watch them both play football because they will soon retire. If we talk about the squads of the two teams, without a doubt I want to say that Real Madrid will win their 15th trophy. However, in the final match anything can happen even though Dortmund in the domestic league is not very good, but in the UCL the team is full of surprises.