If you're asking which card is better, the 680 is almost 50% better performance wise across the board versus the 580.
If you're asking if getting a second 580 or a single 680 is better, definitely go for a second GTX 580. 580 SLI trounces a single GTX 680 and since the 580 is slightly cheaper (not by much though) it doesn't make any sense to go for a single 680 right now if you already have a 580 and an SLI capable motherboard.