depends on u also.

k810 more elegan, sleek and niceer design
k850, bulky, and kinda "alien" design haha.

up to wat u use it for and ur budget. if u reallly want a good camera, then invest on camera or nokia maybe since the lens is not bad. but then again..if ur a sony fan. one is 3mp and one is 5mp. y dun u compare ur friends or whoever has it?