If you're keen on the green team, then I'd vote 670, as TG says, good oc headroom. But, I'd personally go for a 7950 'cos they're (with the state of current drivers), quicker than both in my experience.
Comrade Woody: "I dunno, whores are a bit like cornflakes, you always regret buying the cheap ones."
small(ish): i7-5960X / X99E-ITX/AC - Bitspower Block / 32Gb Ballistix / Titan X (P) / 512Gb Samsung 950 Pro NVMe / 250D
tiny: i7-4790K / Max VII Impact - EK Monoblock / 16Gb Ballistix / EVGA Titan X (M) HC / 256Gb SM951 NVMe / nCase M1
htpc/vr: i7-6700K / Z170i / 16Gb Ballistix / EVGA 1080 Founders Edition / 128Gb SM951 NVMe & Sandisk 960Gb SSD / RVZ01