The SA58 is a brand new rifle, built in the US. Every piece is new, and either made by DSA or a subcontractor.
The STG-58 is a DSA receiver with original STG parts, from a real STG type FAL. Most likely it will have come from Belgium or Austria. There should be absolutely nothing wrong with those rifles. While it's a "parts gun," just about every other FAL in the US is a parts gun as well...they're just fine.
The only other real difference is that the SA58 has a lifetime warrantee.