[Cdesja5]

I would say availability in your country, the ease of getting repairs or returning would be important. Can you buy your reader in a bricks and mortar store or only online? This becomes important if you don't like or have a problem with the model you buy... I can't imagine being several weeks without my reader in the event that a problem occurs!
I can't speak about the Pocketbook never having used one but I like both the Kobo and the Sony. I actually prefer my Sony for the user interface and having the choice between using physical buttons and touch screen. The Kobo only has a home button. But that said, the Kobo is a good reader and less expensive if cost is a factor.
PS: you really should look into using Calibre to manage your books/collections!