The main difference between Nozawa's Goku and Schemmel's pre-Kai Goku is their characterization. Nozawa's portrayal is far more faithful to the naive and self-centered character Toriyama intended him to be, whereas the early dubs tried to make him more of a Superman/Anime Jesus figure. There was also a difference in the level of emotion, since Nozawa did a good job of characterizing him, whereas some scenes in the dub prior to Kai really sounded wooden to me, such as when Goku was trying to convince Vegeta to fuse with the Potara and a lot of the dub lines from the Budokai games. The only part where I preferred Schemmel to Nozawa prior to Kai was the SS3 transformation, since Nozawa's screaming in that scene can be quite grating on the ears.
Nowadays it's somewhat reversed, since Schemmel has improved a lot in the more recent works such as the new movies and Super, whereas Nozawa seems to be taking a toll with age compared to her voice acting in the 20th Century, though it still holds up as being pretty good overall.
Basically, Schemmel (Kai and Beyond) >~ Nozawa >/>> Schemmel (Pre-Kai) as far as characterization and overall voice acting goes imo.
