Depending on your own skill level and your passion and knowledge of the fan game.

If I'm correct, you should be able to "generationize" SiN, but be careful not to destroy the original feeling and fun by making bad art.

Original idea requires alot of work, just to get started takes normally 2-3 weeks. It's comonly easier to make fan game, since you don't have to create everything from scartch. Original story, backgrounds and protagonist/antagonist profiles, for example.

I'd say that make fan game.