The 570 based cards have about 100 shaders more than the 560 (384 vs 480), so this works out to about 1 US dollar per shader with the average price difference of about 100 bucks between the two card types. Is the price difference worth it looking at the performance difference?
For me, I'm happy to keep the GPU price per card to 250 or less, so this seems like a fair performance compromise for 100 buck less.
I see the top single 570 based client produces about 644k rac per day. So you can see the effect of the extra shaders. Roughly 30% better than my slightly OC'd 560. Thats what your 100 bucks extra buys you.
I've been running a mildly overclocked MSI TI 560 for about 2 weeks now. Its been netting over 400k + p/day. I thought it was a good deal for about $100 bucks cheaper than a 570 based card. I think I'll buy another.
See primegrid host info below:
http://www.primegrid.com/show_host_detail.php?hostid=139856
Here's the card info below bought from Newegg:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127565
