If inspiration struck for a new story I say go for it! But don't completely give up on the other. :P

Listen to your gut feeling

^ What he said. ^__^

That's why I never finish anything. :S But I've found focus on my main story.

Sometimes I get to that dilemma where I am writing one plot point, but somewhere in the middle of writing I get a few more ideas of how that could`ve gone. And then I kinda write drafts for each of them to see which ends up being my fave and implement in the base story. But sometimes one of those other story paths inspires me to make a completely different story, based on that idea. Maybe sounds weird, but that really helps me to write both - original and new story... and there are chances of me inspiring myself for even some other story or spin-off from the original and/or new one. And that could be with completely different approach and genre.
So, I think, go for whichever you have an inspiration on writing and chances are, later the new story might inspire you for some fresh plot twists in the other story. :)

That happens to me sometimes too. I usually go with the newer story just to get some creativity flowing. Once I’ve finished a few chapters or so I’ll go back to the first and I have a bunch of new ideas!