I had a DX because I read a lot of pdfs that display better on it. Then I got a K3 for general reading when it came out. And to be honest, I almost never open the DX now. The K3 is so much lighter and easier to carry around and hold.
I'd start out with the K3 and see if you feel a real need for the bigger DX.
Re: protection - get one of the harder-backed covers like the
Amazon lighted cover. They really do provide a lot of protection. My kid has kicked my K3 off the armrest/table/chair on numerous occasions and it's been fine.
Edit: OOPS! Just saw you ordered a K2! Never mind my advice - enjoy!