a friend of mine thinks you should always finish everything you start just so you have experience at finishing things as well as starting.
for me i wonder, can you not work on them both at the same time? or start the new project and see where it takes you thenm go back to the first? if you've lost motivation with it, i believe it does show in the writing. it will seem dull, will lack the depth that you probably would put in if you were more into it, because you're rushing to get it out of the way.
and i tend to write far better when i have no idea what im doing
when i don't know where the story is going, and just let it take shape itself. i guess it depends on whether you work better with a detailed idea
personally, go for the new idea. try it. if it doesn't work out, and you can't develop it, you can ditch it and go back to the original. if it does, then you can work on that and when that's out your system you may feel more inmspired to do the first project again