well, i've never written a novel, but i've written some short stories, and i find that writing your movie ideas out as traditional prose can go a long way in helping you flesh out the details of the world, the characters' mannerisms, and the overall tone.
but you're talking about taking a novel idea and turning it into a feature. if you are confident you can write the story as a well-constructed novel, then i would go that direction and then adapt your own work for the screen. again, when it comes to selling ideas, it's easier for a no-name writer to sell a novel than it is for them to sell a screenplay, depending on who you know. if you write a script, it's easier for a producer to come up with things to add. if you write a novel, there's gonna be so much meat there that they will subtract more than they add.