As writer of both, fanfic is easier to write because the world, character and rules has been established so you just need think about the storyline (As long you do a proper research). Meh, some fanfic writer even choose to follow close to canon event without making any major change, except putting their SI character as the part of main cast.
Meanwhile for original fic, you wrote everything from scratch, easier get writer blocked due to the fact you need established everything on your own-- the rules, world building, character, storyline, etc. Just need more effort to write than fanfic for sure.
I prefer writing original fic (the fact I only write 2 fanfic out of 6 here) since with original fic you don't have limitation and free to do whatever you want with story itself, but fanfic are sure easier to write.