I'm not sure I would take at face value any numbers presented by any of the "Big Four" eBook sellers. They tend to get rather creative when declaring the size of their stock. For one thing they tend to include public domain books which are available elsewhere.

Having said that, if I want a book on history I'm much more likely to find it on Amazon than the others, and Sony is in a distant second place. I can only speak for histories as that is 75% or more of what I read.

As far as should I buy at any particular store I say, "Hell no"! I'll buy wherever I can find it and from the least expensive store. Brand loyalty has no place in a repressed economy. But truth is, compared to print books there is still a small selection of the type of non-fiction eBooks I generally read, even when I look at all the stores.

And the quality of the typical eBook is way lower than the typical PBook, mainly because publishers have failed to grasp the idea of reflowable, user customizable text. Doing a quick and crappy conversion of a PBook to eBook and then charging me the same or more for the eBook as they charge for the PBook is unethical, IMO. If they are going to charge the same or more for eBooks then they should spend as much time designing and implementing them so that they look as good on an ereader as they do in printed versions. In short publishers need to be educated on single sourcing. If they were smart all future publications would be single sourced and permanently saved so that subsequent editions whether digital or printed could easily and quickly be made.