As much trouble as I have sticking to it at times, I believe firmly in the idea that, at the stage I'm at right now, finishing things is far more important than making sure I'm working on the "best" idea, so I try not to abandon things until at least finishing a first draft.
Definitely not novels (at least not yet). I abandon my short stories sometimes. I try to tell myself I'll come back to them later though
My NanoWriMo novel was a first-person sort of science fiction thing about what
really happened to Atlantis. When I came up with the idea back in late October, I was really confident about it. Now I'm 20k words in (I failed Nano horribly) and I'm not so confident anymore, and on top of that I came up with an idea for another novel that I'm itching to write. No idea if that's because this idea is better, or because it's new, or simply because it's different (several third-person viewpoints as opposed to the first-person viewpoint of my other novel).
So what am I doing? I'm working on both of them at the same time! I realize I'm not experienced enough to know which of them will be worth anything in the end. Maybe neither will be worth anything. Maybe both will have merit. Hard to say until I get that first draft down.
Also, jumping between two projects helps immensely with writer's block. I just pray that I don't come up with a third idea for a novel before I finish one of these.