I would say err on the side of finishing, because it's all too easy to always find a reason to give up somewhere along the way. If you need to, think of this solely as a learning project, one you might never show to anyone, but that you're going to complete, if only so you can have the experience of seeing a writing project all the way through.
Once you've made it as good as you can, you can re-evaluate if you want to try to place it somewhere, or just bottom-drawer it. But keep in mind that many wonderful pieces of writing passed through an ugly-duckling phase --and also that writers are often poor judges of their own work.
To be honest, if you read enough from prolific authors, you realize they don't bury their mistakes, they publish them. For instance, prior to writing his masterpiece, Lolita, Nabakov wrote a much inferior book (in Russian, translated variously as Camera Obscura and Laughter in the Dark) with some of the same themes. It's probably most charitably read as a rough draft for the later work. But it was reasonably successful, translated twice, and even made into a movie.
