Both are inexpensive to buy and cheap to run though the warm type has higher running costs because of the extra electricity needed to heat up the water.
A more sophisticated type is the ultrasonic humidifiers uk which uses high frequency sound waves to convert water into mist that can be pumped out into the room.
If you're serious about humidifying your home you might want to consider getting a central or whole house humidifier. This uses a central generator to create moisture for discharge from vents all over the house.