I don't think there is any huge difference. I feel ATI is Better. This shader model 3.0 and 2.0 was also compared by digit in earlier graphic cards and Nividia was rated superior because at that time ATI had only 2.0. I think it was a bluder because in practical there was no noticeable difference.
Onboard always better from ATI. I believe ATI is coming up with RX 1200 onboard on 965 chpset mobos.