here are two sample pages:http://www.guru3d.com/article/his-radeon-6970-iceq-turbo-review/7http://www.guru3d.com/article/his-radeon-6870-iceq-x-turbo-x-review/8
pay close attention to how the card power consumption is calculated at different websites. some sites use card power consumption, some use total system power consumption at the wall socket, some use power consumption delta using another card as reference. most review sites turn off power saving and protective measures to measure maximum power consumption.
imo, the 6870 has lower tdp and lower load power consumption. so if the rest of the system specs are identical, the 6870 will use less power.
it's good to be considerate to your parents and power consumption.