The size difference combined with that incredible explosive finishing ability at the rim distinguishes him from Thomas. This teenager's upside is virtually unlimited. Using Westbrook as a future potential comp seems valid. Whether he fulfills that comparison. who knows?
His game isn't really much like Westbrook's though. Westbrook is all about raw explosive athletic power. Fultz, while definitely an elite athlete, is more about skill and precision in his game. His most elite athletic attribute is probably his precise body control and balance.
Think of scorers more like Harden, DeRozan, Pierce and Kobe.