I would do science-fiction. I've grown up with both, and actually probably read more science-fiction than fantasy. That's probably not by choice, but more because of the author's I got into did more science-fiction than fantasy.
I have not experienced it. This is probably because I am a "plantser". I make an outline of the story, including the parts that have to happen, and where I want characters to go (evolve or devolve). That's the plotting. Then I let the actually bits in between play out, pantsing. But I know where they have to go, so I just play with the possible ways in my head. My advice would try to work out the basics before you dive in. Writer's block usually comes when you don't know yet where the story is going.
