I pick the one that's least stressful, most fun, most interesting, most visually pleasing, etc depending on the situation (deadlines, mood, etc). Maybe it's just me, but I can usually get a precise image in my head of what I want to happen here while playing through the game. It usually comes to me when I'm doing something else, though.
MSGhero
http://gamedevelopment.tutsplus.com/articles/should-you-polish-your-game-or-add-more-depth-and-features--cms-21155
Or I ask my writer.
BoMToons (Updated )
It's not so much a question of "polish" vs. "new features" as it is, how exactly to lay things out, visuals, ordering, pacing, control feel, etc.
I guess there is some "what features do I want in here" but I don't feel that pulling away from the level of polish...
It's kind of like: "I need something interesting here... out of everything I COULD do, what do I WANT to do?"