Technically speaking, Shelby over Pond.
Except for very few experts out there, there aren't too many people who can tell the difference between the two blocks once installed. If I remember correctly, the Pond block weighs 110 lbs and the CSX block weighs 125 lbs. So the weight savings is virtually negligible to most of us mortals.
I chose the CSX block because of the
perceived added strength, better oiling, bigger potential for cubes (I've seen 527ci) and the dollar difference at the time I was buying was only $500 more for the CSX block.
You can't go wrong with either choice. Both are fantastic blocks.