After experimenting with a lot of apps over the last year I'm finding that Scrivener on the Macs and iA Writer on the iPad suit the extremes I like ... complex (for the whole thing, a long and complex book) and simple (for just writing).
While you can do simple in Scrivener with the distraction-free page I've become used to using the iPad with iawriter like a typewriter, as writing is all you can do in this I don't fix spelling, typos or edit in any way, whereas on the computer I do stop and shuffle commas around or whatever.
Well, I'm Virgo (also known as Anal, according to some younger family members) and so analysis, detail, editing and organisation are natural habitats, which is all very useful once the essence or content is there, whether fiction or non-fiction, and totally useless when it isn't, and a sure recipe for writer's block, in my experience.
So the simplicity -lack of control and options -in the iPad writing system is partly psychological for me.
Strangely, limitations have this reverse effect for me, I've had the same situation in music systems ... A very limited but quick and intuitive little 8-track sequencer inside a keyboard versus massive Protools and myriad plug-in apps inside a computer.
The fire, water and air signs among you, who don't need to set limitations to release margins, are probably rolling your eyes ... Ha ha.
Haha, well as a fire sign, I identify with that