I buy from the cheapest place. Period.
I wont be locked into a device. I cleaned up my books in Calibre and convert them to mobi and epub. I then send the mobi to my personal documents cloud.
Using The Kindle Personal Document Cloud you pretty much get the same benefits you get from Amazon bought books. Syncing Last read, bookmarks, highlights and notes and it stays on the cloud so you can download it from any Kindle or Kindle app.
There is no real advantage over either store even with a Kindle.
It's just who has the lowest price.
