If you have a quality (Seasonic, Corsair, etc,) 600 W PSU there should be enough power for either of these two. I think that the Gigabyte SOC is the better choice because of the dual fans. but it's a close call. Both GPUs are (presumably) reference with the difference being that the Gigabyte is factoiry over clocked. I am running a reference GTX 470 OC'd and can OC with anyone with an aftermarket Zalman VF3000 cooler. If you plan to over clock then you must mind your cooling--heat kills.
Now, if you don't intend to OC then the Gigabyte will give you more performance and keep you within warranty.
BTW I picked up my GTX 470 (lightly used by a non OC'er) for $162 shipped and put on the Zalman cooler for $65. It blows the doors off the GTX 460. and runs cool.