For such things I invariably use WriteRoom. I find formatting and futzing with a web app to be built in procrastination mechanisms. It's generally trivial to spend a few minutes copying and pasting once the actual text is done.
WriteRoom is nice—it's also one of the first distraction-free writing app—but i prefer to use Byword. I like to combine paragraph focus with full screen for distraction free mode. Also it support markdown syntax, that for me is a must have for a writing app.