Of the two the Kindle is the newer, sleeker and more responsive and my prefered reading device.
The Cool-er natively uses epub while the Kindle uses Amazon's azw and mobi formats natively.
Now I have a fair number of books in epub format – most came from project Gutenberg meaning that I can simply re download the if I want to reread them on the Kindle. Or I could convert them using Calibre.
The problem really comes with the DRM-epubs obtained legitimately from other ebook vendors. In technical terms the solution is simple – I could take the files and process them to remove the DRM and then convert them appropriately.
I havn't done this as, whatever my opinions about DRM, it's unethical to break the conditions of use. But this does raise an interesting point:
You don't really own an ebook. You rent it on a long term basis. This means that if you change platforms say from epub to Kindle you have to re rent any of the content from a new supplier.
Ninety percent of the time this doesn't matter as most people don't reread most of their books, or if they do, fairly soon after acquiring them.
The problem is, where do you stand if your reading device dies (and remember that in the case of the Cool-er the manufacturer has gone to the great stock market in the sky) or the ebook vendor has likewise ceased to be (as in the case of Borders Australia) or stopped selling ebooks (Bookdepository)?
Yes, you still have your content, but you can't necessarily access it in your preferred manner unless you break the conditions of use.
This is different from a book – once you buy it, you have it, you can lend it to anyone you wish, or you can sell it if you no longer want it.
With an ebook that's not necessarily the case. And when I look at my collection of a thousand odd travel and history books I begin to wonder about what would happen if they were all digital and I was to lose access to them ...