If you read the Onyx forum you can see that the M90 has a lot of problems at the moment. Can they fix that with firmware? That's the question. The hardware and design is better than pocketbook.
The 903 is more stable at the moment, and have more functions, more support, bigger community.
I have one 602 and Im happy with it. I was going to buy a 9.7" and like you I was thinking about the onyx or pocketbook. Them I decided to buy the pocketbook pro 602 and wait before buy a bigger one. I can read pdf (scientific) in landscape, it's not perfect because sometimes I need to see the all page and that is not possible.
Is not a easy choice. If you can, wait a month or 2 to see if the onyx get better, if not go for the pocketbook.