Get the OC version. This one allows you to undervolt and will result into less power consumption while keeping good hashrate. Now people say they get over 760 hashes per second per GPU. I was not able to get that stable. The best I could get from these same cards is 740 which is not bad as well.
If you can get the Sapphire Vapor-X OC 29 280x. Not too big different compared to these you have mentioned and the cards are built to last long time even though mining means 24/7 usage.
Got the OC, but I only get 710Khash/s with Voltage at 1050-1075. Which settings do you use?