What's the difference of Forestry and Contentful? I've heard good things about Contentful, but I haven't heard of Forestry.
Forestry.io is a Git-based CMS that commits content changes to your repo (and optionally builds/deploys your site). Contentful is an API-based CMS, that stores your content and requires you to query their API during/after build time.
So you would use Forestry if you're hosting static HTML from CMS, and Contentful for when you need an API for the CMS.
(disclamer: I've never used CMS before so I don't really know how it works)