I've been monitoring Sony and Amazon ebooks for over a year and reporting biweekly status here:
https://www.mobileread.com/forums/showthread.php?t=16810
The short answer is that Amazon (about 210,000) has almost four times as many ethings for sale as Sony right now (about 55,000). The Amazon total includes probably a thousand or so multiple copies of public domain titles by various publishers. It also includes about a thousand or so "self-published" titles that Sony doesn't sell. And it includes newspapers, magazines, and blogs that Sony doesn't offer. As far as strictly ebooks, there's no question that Amazon is selling more titles than Sony. There's a strong overlap but there are few titles that Sony sells that Amazon doesn't and very many titles that Amazon sells that Sony doesn't. The thread above includes info on at least two specific subjects that have been compared directly.
In October, Sony publicly promised that they would have 100,000 ebooks for sale by the end of this year. As yet, we've seen no sign of that happening.
As far as epub is concerned, at the moment, that's a tiny fraction of the ebooks available for purchase. I personally don't like how epubs are rendered on the Sony Readers with the current firmware and won't buy any more until that's fixed. I also don't know if they have any plans for such a fix either.
Mobipocket to Sony? I'm personally avoiding that avenue if I can. But if you buy Mobipocket ebooks and convert them for the Sony, the landscape changes, probably dramatically. The main issue, though, is that there are many Mobipocket ebooks that are available only from Amazon. And, at least at present, I know of no way to buy one of those ebooks from them without owning a Kindle.