The Kindle(s) are not open , in some regard :-) ... and sell like hot cakes. But you can find any type of book at extremely good prices. Actually, B&N tablets allow you to side load books via SD Card. You have no SD slot in Kindle(s)
Tablets and ereaders are now pretty much the same, Vey good quality overall. With few exceptions, you get similar hardware among companies. What really makes a difference, in my opinion, is App Store and the tablet or ereader price.