One factor might be how good the store is for browsing...I really like the Kindle store, it usually has more books than anyone else, and it's really convenient to buy books...and I've definitely noticed an increase in my spending on books, even though the books themselves weren't expensive.
So, you might even want to get a reader with a worse shop, and get more free public domain stuff :P
I agree with the above posters though - Get the reader you want based on the reader itself, get calibre and you're fine
