The thing with Forestry is that you get to make changes to the fields using the UI.
While using Netlify means you have to write some code to define your fields and might have to look at the docs so you know the syntax and choices available
Once you have it set up, either is fine for your content managers. But if the needs ever change, as is common in business and software, then Forestry will let an admin change the fields without knowing how to code.
Netlify let’s you do advanced stuff like determine what the fields look like. In the one project I used, there were previews on the right of the content on the left.
I’d say try out Forestry first as it has lower barrier to entry and spend time on Netlify CMS too if you want and have the patience. Then make the call
Or find some article or video which compares both.