When Amazon launched the Kindle, they had 90k titles. At that time, none of the titles were from PG, and hardly any were from the public domain. All of the titles were from commercial publishers.
(BTW, PG hadn't started converting to Epub and Mobi until long after the Kindle was released.)
The iBookstore, on the other hand, has around 60k titles, one third of which were converted by PG in an automated process. A third of the iBookstore is crap, basically.
It's not fair to compare the 2 ebookstores, really, because Amazon had a much harder position. Apple got to follow in Amazon's footsteps. Amazon had to convince publishers to try ebooks. All Apple did was enter an established market (like they usually do).
In fact, it would be better to compare their respective business models. Amazon will do business with anyone. Even if you only have one ebook, Amazon will sell it for you. Apple won't. I found out from a writer I know that Apple won't accept ebooks from anyone but publishers and aggregators (Ingram Digital is one).All of the aggregators charge an upfront fee for their service, and that will probably discourage most authors and small publishers. So at least half (if not more) of the Kindle Store's 400K plus ebooks will never make it into the iBookstore. How exactly is this supposed to be a serious competitor to the Kindle?
P.S. The fact that Apple insist on aggregators should tell you something about what they really think about the agency model: it's a sham. Apple are taking a retailer's cut and forcing the small fry to give up a distribution fee to the aggregators.