Portable heater: should I go with a lower watt or higher?

I know that the higher the watt is, the more expensive the electricity bill is going to be, and that concerns me. But will a 700 watt heater be sufficiently warm, or would it be best to go for a 2000 watt heater? The 2000 watt heater is extremely cheap, and the 700 watt heater is more expensive. They both have 3 heat settings. What do you recommend?