I love Hahn Photorag Baryta (probably my favorite), but it uses PK ink, so I've had to stop using it since the PK head in my R3000 died. I'm planning on replacing my R3000 with a P800, but til then I'm stuck with papers that use MK.
As for OBA's, I was never THAT concerned, as I thought that they just faded evenly, but I recently heard that they fade in splotches, which brings to mind the foxing on old albumen prints etc. I work in Preservation as well as being a photographer and I'd like to avoid that.
I've not seen Canson Platine, but I'll definitely take a look.
Stephen Jennings