Is a matter of usage more than source.
Many performance tuners like an iron block to have been thermal cycled to provide a more seasoned surface.
Many manufactures will also store the engine castings and allow them to rust before machining to get the same molecular alignment as a used block.
The builder and the tolerances he uses, and the parts he installs are the real deal.
You can get a dang good engine from both, but if used beyound the design it is junk looking for a place to happen.
Here is a dang good Stang Dyno run.
