K790a does have 850.
I've owned both. Currently I own a D900. The K790a is BY FAR more customizable. The camera is better (IMO) because you can push the camera button halfway for AF to focus and then fully press it, like on a digital camera. With the D900 you just push the button and hold the phone still for a few seconds.
I like change so I'm always getting new phones. The K790a was probably my best allaround phone. I like the D900's size and screen more, as well as the memory card format. It really comes down to what you're looking for in a phone.