This is just my opinion mate, but for me the difference is that Williams does everything at greater speed in comparison to a younger Zaha. The highlights below of both players demonstrate to me the difference in speed, tempo, intensity and quicker directional shifts, which are all in favour of Williams who looks a more electrifying wide forward at the same age. Williams is also playing at a higher level in Spain compared to Zaha, who was playing in the English second tier at the time.
Williams
Zaha
And from watching Williams play and forming a opinion via the eye test, the next step for me is to have a look at the statistics to see how he compares to his peers in key areas in his respective positions on the left or right flank. And even the statistics are painting a pretty picture as far as his key attributes are concerned playing as a winger/wide forward. The other player mentioned in the tweet below is Johan Bakayoko, but he's playing in a weaker league, so it's difficult guage how good those stats are until he makes a step up to a stronger league or has a run of games in European Cup competitions against quality teams. But Bakayoko is naturally left footed and Williams is naturally right footed, and I wouldn't be against signing both, because stylistically both players are potentially a throwback to the wingers of yester-years and I think we could definitely do with some variety which is lacking throughout the team where a lack of physical and athletic ability is a issue.