5D2's low light performance (low noise at high ISO) is greatly better. These were all taken with 5D2:
<a href=http://bbs.wenxuecity.com/photography/838054.html>Performance</a>
<a href=http://bbs.wenxuecity.com/photography/793688.html>Gymnastics</a>
If using 1D3 to photograph the gymnastics photos, the noise will be quite visible even at this small size.