I went from a Sony 505 to a K3 and don't regret it at all. I just love the ease of being able to send all my books via wifi to my K3. As much as I love Sony, the lack of wifi on their smaller models is a real dealbreaker for me.
Being able to subscribe for wireless delivery of magazines and newspapers is also a feature I can't miss anymore. I cancelled a magazine subscription a couple of years ago mainly because I hated having to deal with the waste of paper. I decided that I would only resubscribe when I could read an e-version. It's one of the first things I got when I received my K3.