I use both. iA Writer is my definite writing app. It's where I write any bit of non-trivial Markdown, to the point I wince when I have to use an old-fashioned word processor. Drafts is exceptional for dispatching little bits of text to other apps through its ridiculously powerful integration and scripting. For instance, if I think of something I want to post to Mastodon, I press opt-F2 to open Drafts, write it, then press F2 to send it to my server.

Writer is a much better text editor IMO. (Drafts is really good, but Writer is stupendous.) Drafts is much better for interacting with or automating other apps. They're both in my dock.