Author here. The main difference is the target use case. I'm far from being a Pure Data expert, but it seems more aimed towards music/media generation.
Flyde on the other hand is aimed to be used in the application layer of modern web development.
The model seems quite similar to a subset PD, with only object and message boxes. Objects can have only a single output port. (edit: I was mistaken, they can have multiple ports, though as far as I can tell there's no port-sequencing rules a la PD). Objects don't seem to be addressable: the only messages that can be sent are via the visible connections.
The type system seems to be directly leveraging the host JS type system, but the docs seem a bit sparse on that.
The documentation on control flow is empty, so it's not clear what the model there is precisely, but it apprears that the entire system is message-based, given the way the Hello, World is written: "output hello, wait a bit, output world".
The documentation suggests that what's novel about Flyde is the integration with other codebases, but it's JS/TS so it requires that whole enviroment. PureData is embeddable in any C application, so any language with C FFI can integrate.
