ORF VS DSA
Definitely spend some time at falfiles.com before making this decision. Also don't limit yourself to just these two companies. With that said, My ORF was probably the best $500 I've spent. It needed nothing to work fine other that a little "breaking in" you might expect from any auto. That is to say the gas ring was a little tight when I got it and the action was a little stiff for the first 25-50 rounds, after that it settled in nicely and funtioned fine with most milsurp ammo. (Stay away from Indian and CAVIM ammo). With irons at 50 yards I could get sub 1" groups, at 100 they went to 2+ or 3 but I'm not that great with irons. The ORF I bought was a "parts gun" on an IMBEL receiver.
I think the DSA STG 58 is a "parts gun" with some new USA manufactured parts included (as required). I think the SA 58 is a totally USA manufactured gun (thus the difference in price). I am just going by memory and may have these reversed or not completely accurate. The DSAs are quite a bit more expensive but also have more options.
There are some good smiths out there who can probably give you exactly what you want for less than the price of a DSA, but it may be hard to beat the value of an ORF, I'm not sure how authentic you want to be though. I went with ORF because I wanted a shooter that was reasonable.