Project riff most closely compares to OpenWhisk and in fact both riff and OpenWhisk contributors work together on Knative. Of the two, riff has the advantage that it came along at the right time to be based directly on Knative without to be "ported", so to speak.

PFS might be more fairly to commercial plans announced by other vendors to repackage OpenWhisk.

Disclosure: I work for Pivotal, though not directly on PFS.