I started tracking Sony's offerings two years ago and have on a couple of occasions tried to make a careful comparison of them with Amazon. The thread is here:
https://www.mobileread.com/forums/showthread.php?t=16810
Basically, the total number of ebooks (not blogs or newspapers and being careful to avoid duplicates) is well more than two to one in favor of Amazon and that ratio is steadily increasing. If you pick a particular subject or genre, you're much more likely to find what you want at Amazon that at Sony. Where the difference is the smallest is for new titles where the two vendors seem to be more closely matched.
It's hard to make a general statement but I'm frequently annoyed by finding a title I want to read that is available for the Amazon Kindle but not for the Sony Reader.