I think both the plotlines have potential.
Still, I have to say that personally, I believe it's not as much about the basic idea of the fic, but what the writer will DO with it. Though, I can understand wanting to bounce ideas and have some confirmation from others about the basic plot idea in order to kick-start the writing process. :3
But at the very core of it, your own opinion is what matters the most. You should choose an idea which YOU will find interesting. So that you will want to keep on writing.
The idea is but a corner stone - it is crucial for building up the story, but what kind of a story you are going to build in your own hands, and that's the fun of it!