neither. id get a 670 for now and then when you need more performance, add another one. the performance difference between the 670 and 680 is not worth the 100 dollars. the 670 id get is the msi power edition. slightly cooler than the asus but it has overvolting that no other 670 has (unless using a ROG extreme board or a EVbot)
id go 690 if that is what you are looking for. it uses less power than 2 680s, it is more quieter, and with a decent case with good airflow, the 690 is cooler than 2 680s.