It all depends on how and where you read, I usually read at night in the bedroom and a backlit iPad works great for me - my wife travels a lot and uses her K3 in well lit areas (mainly airports!).
While I have used the K3 and do like it, it just doesnt fit my reading habits and vice versa.
What are you're reading habits? Maybe a backlit kindle (or other eink) would work for both of us.
Another factor is how techy you are, I liberate my purchases (and I do purchase) and can convert as required - for a simple life, the K3 wins here due to the number of available books and ease of purchase/ download (despite it's 'rental' model).
I guess it's all down to how you read and what you want to do with your purchases - hope the ramble helped
