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Out of the kids that i have asked about this at least %60-%70 say yes that they like the fiona and cake series better than the finn and jake series. I like this series because it turns the world on it's head and give the show a whole different meaning.
It doesn't help that the two genderbent counterparts barely appear on the show, but Fionna and Cake aren't very interesting in their personalities or adventures. Finn and Jake, on the other hand, have been so eye-catching that it has kept people watching the show for about a decade now. The adventures of the two original characters, themselves, are debatably diverse and constantly changing. Examples are Finn trying to get revenge on his father for all the pain he caused him, Jake dealing with his death in the near future, both of them fighting over what they should do eventually leading to them physically fighting and having to make up. Fionna and Cake just do what they are told and show no personality, not even copying the original two. They have only had two adventures, one of which has had them fight a villain.
Firstly, Finn and Jake are the central characters. Without them, most of the show isnt even possible. Secondly, they are better role models for teen boys, whereas Fiona and Cake are for little girls, not that it's a bad thing. Adventure Time can appeal to anyone, so I think its cool there is female versions, but Finn and Jake are funnier and defiantly more layered...Like ogres.
No, Fiona and Cake are inferior to Finn and Jake. Adventure Time is all about having awesome adventures with you awesome and crazy friends. Fiona and Cake while adorable are not awesome swashbucklers like Finn and Jake. Fiona and Cake are more interested in romance whereas Finn and Jake have a balanced interest in romance and adventure. Finally, Finn and Jake are hilarious.