Pods is any type of content you can imagine with any field you want. ACF adds fields to existing content types.
Pods goes beyond that though, though it’s primary focus is Content Types and Fields, there’s a Templating Engine {@name} was born on {@birthday}
and a Unified Theming API which lets you code across any content type, whether that’s a Post, Taxonomy, User, or Comment — using a WP_Query like function that works with them all and does the hard part for you.
Pods also has Wildcard Page URLs, so you can create pages on your site like people/*/friends/
and code it to associate those pages to your people content type, and list their friends. (many more things you can do with it)
ACF lets you add groups of fields to any existing content type, Pods has a grouping API too, the UI for it is coming in Pods 2.1 due out this December. What ACF does differently with it’s groups though, is that you can set location based rules for the groups, so they only show up for a specific Post ID, Taxonomy, etc. We’re looking into adding certain parts of this kind of logic into the new group UI for Pods 2.1, but that hasn’t been fully built out yet so it could either be more or less than what ACF currently does.
ACF also has a Loop field type, which can be useful for content that needs to be flexible enough to have as many as you want. Pods 2.1 is slated to have this type of field but it will be more functional than just the normal loop field most people have seen.
We’re working on a comparison table between Pods and many other plugins, watch for it in the next week or two. I’ll do my best to remember to come back here and reply with the link.