The ATI HD 5xxx series is rumored to be out earlier, but we do not know. As for performance, ATI might beat nVidia because they have been employing a lot of the tricks that are in DX11 before nVidia, but no one knows. It is likely that we will see a similar scenario to what we have now, very close performance.
nvidia will probably win in DX10 for single GPU's if it follows the leaked specs, but in DX11, it will depend on if they take their time to perfect it and release it in Q2-4 2010, or they rush it and get it out in Q4 2009.
however ATi will have the 5870 X2 ready for release around the time when the top-end nvidia card comes out.