If you ever plan to get two, go with the SLI option and the GTX560. AMDs drivers for crossfire will give you high fps, but you'll start to see intermittent flickering video while gaming with crossfire. This is a non-issue in SLI.
I find more value in a flawless implementation of a 2-card setup rather than paying less for a flawed implementation. It just depends on what you're willing to compromise.
/quotemsg]
I don't think making that kind of generalization is fair or accurate. No doubt that some users may have issues with crossfire implementations, just like some users will have some issues with sli. If there was a measurable problem with crossfire, it would be mentioned in any of the dozens of crossfire reviews out there, or crossfire vs sli articles out there. It's not- there are no studies on it or numbers referenced here. I don't think I've seen a single mention of more prevalent issues with crossfire than with sli in any review or article. If there's some basis for the comment, I'd be interested to see it.
Currently crossfire scaling is simply better than sli. A pair of 6970's outperform a pair of 580's that cost about $300 more. There may be reasons why someone would choose the 580's over the 6970's (like having a personal poor experience with an ati/amd setup in the past) and that's fine. However, personal experience should not serve as a barometer for a product line. Both Nvidia and ATI make great products- making claims to the contrary (from either camp) should be backed up with some sort of facts.