From what I've seen of Viz (omitting the 10x Oozaru multiplier, taking many liberties with lines etc.), I'd say Steve Simmons' subs are much more accurate. Viz is only slightly more accurate than the Super dub is.
As Kenshi said, Viz omits too many details (eg. Raditz's specifications on the Saiyans who survived Planet Vegeta's destruction) and adds a few pieces of unnecessary dialogue too. The Steve Simmons subs are far more accurate when it comes to information in which both continuities line up.