For fanfic, things usually stall out for me for real life reasons or perhaps reasons of over-commitment to ideas and something has to give. In those cases I go wherever the muse dictates. When it comes to original stuff though, I think the choice is a little tougher. I'm bad about stalling in the middle of stories anyway so it's a matter of whether or not I think I can re-outline to make the story work in the second half, or if something is salvageable for another project but just not working for the current one. I tend to write original stuff in layers, with lots of different pieces so I recycle a lot.