The GTX 570 is the better of the two. When you SLI the the 570's you will have one fast rig. But you might want to wait for the Kepler whichis Nvidias new series. The GTX 680 might be out in February with a good price tag. So it depends on what you want to do.
The 448 core performs almost right with the 570right out of the box.
If yOu want to spend.the money then hands down 570.
If you want to save a bit and get almost exact same performance the 560ti 448core with 1280mb ram is the way to go.in my book
If you are thinking about the 560 448 core and there price tag is about 290 dollars. The 570 is about 350 dollars. Now the rumour has it the GTX 680 is going to be around 399 dollars.
It is up to you eveybody on here has given nothing but very good answers. It all depends on how your budget is and what you feel is right for you. No matter what way you go you are getting a nice card nomatter which way you look at it. Good luck