Honestly, go with the HD 6xx. Having heard both the K712 and the HD 650, the Sennheisers generally just sound much better. The AKG house sound trends too much towards the high end, being extremely analytical and cold. The 650s- which the 6xx are essentially a clone of- just sound better because they're warmed up around the bottom where the K712 doesn't have any presence.
To be completely honest, the K712 might be .02% better for super sound-whoring, but generally speaking the HD 6xx will be 100x better for literally everything else. Not to belittle, but especially since you asked the question, I'm going to assume that you're on the casual side of audiophile, where the 6xx's signature with more warmth in the lows will appeal significantly more to you.