Glad I could help with the decision. The PB626 is certainly worth its money. You can run Koreader on Pocketbook too and use "my"
selector app to switch between the two pdf-readers. The slow start of the bigger pdf was a bit of a show off but intercepting by using a button or screenclick seemed to help. I tried it again now and it did start within 10s while clicking the foreward button often to get it out of the sluggish behaviour.
Edit: The only thing I found better about Koreader is the option to change the contrast in pdfs.