How many wattage to cook using a rice cooker?

I don't think there is enough information to answer that. A 1000 watt device certainly has the capacity to use energy faster, but if it can be set on "low" (a must for cooking), then it can be just as efficient as the other one. If at first it boils the water faster, that doesn't mean that it uses more energy - unless you accidentally leave it on "high" too long, and the water just boils away.