joshbarr
It's worth noting that the Wagtail API module is for GETting content at this point – though I hear a more fully featured read/write API is in the works. Wagtail "is just Django" so it's easy enough to plug in the REST framework and build endpoints on that.
I implemented Prismic for a startup recently, and the API is also read-only. It's got some very powerful semantic content markup and versioning stuff, but I found it quite a chore to get more than single nodes out of it. In the end, our content editors didn't like the git-centric "content releases" metaphor, so they moved off to something else.
Contentful has the read/write API thing down, good support, very actively developed. Unfortunately it doesn't support tree-based content, preferring flat collections with relations, though trees are something many users are asking for. As we're migrating lots of sites (off a PHP CMS) that have well-tested trees, we couldn't quite go that far just yet.
tl;dr? No solution is perfect, but Wagtail is making us happy devs, and the community is very friendly. Good luck with your CMS hunt!