dual cooloer vs single, the faster vs the faster but im looking for which one is better in stock and is more reliable.
overclocking will be done but in the near future.
which one do you prefer and why?
Well the 570 is faster... they are pretty much equally reliable and such.
Really it comes down to specific models. I mean, the 570 will always be faster than a 560 Ti, but for overclocking, temperature, noise, and stock speeds - not to mention warranties - it depends on the brand (EVGA, MSI, Asus, Gigabyte, etc).
The 560 Ti 448 is considered one of the best bang for your buck cards out right now. It's based on the 570/580 architecture.
However, the 7850 and 7870 from AMD are really freaking good cards at good price points so don't rule those out.