basically Workmanship.
the tony law system whilst not being stainless has quality welds and joints, a lot better in quality than most other systems.
the ashleys that i have seen have been really thin and lower quality and people have to fiddle a bit to get them to fit, although people have still made good power with them.
have only seen pictures of the simpson exhausts, but they look pretty impressive.
i''m sure they have all done some development to see what is going to work ok on the majority of engines, but you can still make better power using a cheap set than a more expensive set, just depends on what has been done to your particular engine.