If you have the money I would go for it. You will get more stablility with a single GPU, plus lower temps. Just do your research on the different 7970s and find the one the overclocks the best.
The two 560s are about equal to two 650 TIs, which equals somewhere inbetween a 660ti and a 670, whereas the 7970 is more equal to a GTX 680.
Thanks mate , I have already listed my Gtx 560 ti's for sale
And already have one guy to purchase me one of them