I hate writing about toolsets, but I would be pretty happy if I could wave a magic wand and combine the library and publishing workflow of Ulysses with writing environment of IA Writer. Of the two, I generally prefer just about everything in Ulysses, but I do really like how things look in IA Writer.
In the end, though, it doesn’t matter and it is not worth spending any significant amount of time thinking about it. The best writing environment is the one where you actually write.
Everything else—all the trying out of new apps, fiddling with workflows, trying to get everything organized just right within a single system—is distraction and waste.