They are quite different. First I will make a distinction between PAPI and RAN.
PAPI is the Platform API that allows you to administer the cluster. It is not about accessing files.
RAN is Restful Access to the Namespace and is about accessing files. I assume this is what you are using.
The difference between RAN and Swift is that RAN sees the namespace as a filesystem. This means you have calls to do just about anything you could do from a protocol. You can view and alter Windows ACLs, for example. You are getting a Restful view of the cluster.
Swift is a true object store standard. That means that you don't think about things like file permissions. You access objects through an account and the account is the controlling factor. OneFS still enforces file permissions because we have to, but the Swift Protocol itself doesn't see it that way. There are some other significant changes coming in the next major release of OneFS that I can discuss once I see it posted but I will say that things like accounts and user access are quite different as well as location of accounts and containers.
The trick is to use the protocol that makes sense to your workflow. Swift is nice in that it's an open standard but if you still need to interact with the data as if it's in a filesystem, (e.g. you want to view and manipulate permissions), Swift isn't for you.
Bottom line, one is simply a RESTful interface to a filesystem, the other is an true object store.
Hope this helps.