I don't know much about the W610i, but the K800i seems like a good phone, good reviews, good camera, 3G the only downfall is that it is not the smallest and lightest phone.
Some people don't like the SE joytick navigation (I don't mind it, but it is not my preference) the W850i (I think it is) doesn't use joystick navigation.
I am thinking about getting a new phone, I am probably going to have the Samsung D900, if I had more spare money to spend (I am on Vodafone pay as you talk) I would have the SE W850i.