I did some tests a while back (may be able to find it around here). Basically... Out of the last 13 books I purchased only two or three were available in any store besides the Kindle store. And the Kindle store had them MUCH cheaper (often as much as 50% cheaper).
It's not really just numbers, though. Technically, with this Google deal sony has more now. But it's 100 year old books in the public domain... most of the public domain people may want is available across the board. I'm not sure that means anything, then.
As to what most people want... I dunno. I don't read like most people. I imagine best sellers are probably available everywhere. But I tend to read non-fiction, and the Kindle absolutely owns the genre.
To me it is
the issue. I had a Sony reader, and liked it, but I was frustrated at not being able to find ANYTHING I wanted for it, and if I did - it was extraordinarily expensive for an ebook. I still can't get a LOT of what I want, but I can get a decent amount.
It does always suck when you get stuck with a topaz file, though. Doesn't happen often.
The content discussion always has to be couched in "what you read." If you read things in the public domain (classics, etc) almost exclusively - anything will work. If you read a lot of genre fiction (scifi, romance, etc.) you'll have a lot of options across the board. If you read new releases you'll have a lot of options.
But if you read more nonfiction, you don't have a lot of options. If you want newspapers and magazines (and for some reason calibre isn't an option), you don't have a lot of good options. Even if you read a bit more... vague fiction, you don't have a lot of options. So it's really always about what you read, how you're going to do it.
The good news is that on the Kindle... you're not limited much. Prices are better most the time, and very little is available for other formats that is not available for the Kindle.
If that changes, and someone else gets the goods.. I'll jump ship. I'm only interested in content and price. Brand loyalty is for suckers.