It's not fair to compare advertising bucks on an apples to apples basis. Amazon has something on their store below every book which is available for the Kindle (which is most of them by sales volume) saying "Available on Kindle". The cash value of those ads would be enormous, far, far more than Sony has spent I'm sure.
I've resisted the Kindle due to its DRM and whatnot, and up till now, the Sony device didn't have much appeal either. But I'm quite interested the ability to "check out" books from the New York public library and to read Google Books in epub format. Those may be the killer features I've been waiting for.
Many of the Google Books titles are already available for free for Kindle usage from sites like FeedBooks.com; it's quite easy to download those titles over the air.
FeedBooks has less than 3000 'official' books (mostly public-domain titles) and around 600 user-submitted books; over a million Google Books can be read on the Sony Reader. However, the OCR quality of ePub content from Google is spotty. I believe all books on FeedBooks have been proofread.
I'd go with the second lesson. Kindle makes it really easy to buy a new book: you don't need computer or even wifi network, just hit the button.
I wonder if it is a good idea for rivals to create a market with one point-of-entry but different sellers. Think of, well, Amazon for anything but e-books.
On a second thought, such an open market will make price-unification impossible. And that is Kindle's another strong point.
Why is price-unification good? Price-competition is better for consumers.