Surely similarity can't be enough to disqualify an idea. If it were, there'd only be a few books out there. The way the story is told makes such a difference, and especially if there are already enough dissimilarities (and they're the kind to induce rage) and you're feeling ready to go for it - why the hell not? Yeah, tell your story. By virtue of being yours it'll already be different from whatever the other is.

That's a pretty awesome takeaway to get from fic! And all the more reason to go for it... :)