Whats the differense with W and K series? Is it the Walkman player?

Actually I never used the Walkman Player on my W810i and I guess I never will.
About the camera, I want to be able to take pictures but I don't care much if it's a 2mp or 5mp or the zoom. Just want the possibility to take pictures.
Mainly only using it as a phone and have fun with it like themes etc.

That slide of w910i I really don't like =)
w960i seems okey but it's somewhat bigger than w810i...