It really depends on what part of the process you are working on.
My son needs more fluency so we are going back to daily written narrations and will work on grammar and spelling within that activity. That seems to help the most as far as basic writing fluency of all the things we have tried. I just purchased Know and Tell: The Art of Narration and the “Your Questions Answered: Narration” book. Hoping it can help us move to the essay step without too much of the fluff which just muddles his sentences.
Last year he was semi successful with Lost Tools of Writing with another teacher. It was good for brainstorming etc but really a bit too much on the formatting and ideas so he spent less time constructing sentences and often his sentences turned to muddle trying to work through all the other stuff.
We hated IEW. He needs to make good sentences that aren’t confusing and adding all the extra decorations just made things worse.