Many writers ask this question:
Is it better to keep revising my first novel until it garners some serious interest from an agent/publisher, or should I ditch the first novel and write a second one?
Having faced this exact dilemma (as I'm sure others have), I say ditch the first one if it doesn't get some interest after you've given it your best shot. What does that mean? Write diligently, get in a critique group, revise and edit until it shines, then query the heck out of it.
After you've done that, however, I don't think there's any payoff spending another six months, or a year--or longer--working on something that's already proven to have little commercial interest. Sure, you could transform it into something awe-inspiring, but that's unlikely. Remember, we're talking about first novels here.
I believe your time is better spent working on a second manuscript, being sure to use all that you learned writing the first. There's a learning curve every writer must climb, and I think you climb it better (and faster) when you work on something fresh. New characters, new plots, new challenges.
I know the first manuscript I wrote wasn't fit to see the light (it resides, to this day, under my bed, guarded by mousetraps and hairy spiders). In fact, it took me a few more attempts before I had something publishable. With each manuscript, my writing improved greatly (the prose in that first manuscript? not pretty!).
Of course, I've seen/heard about successful books arising from years and years of revision. And if you've only got one story in you, then it behooves you to keep at it. But I think if your goal is to be a writer who will complete more than one book in your lifetime, you need to learn when to cut bait and start a new project.
Just my opinion. I'd love to hear yours.