Like others have mentioned, the biggest difference is defense. The Greek Freak could potentially be a defensive player of the year candidate. The Klynyk is glacier slow and often gets in quick foul trouble.
Check out these two comparisons of Giannis:
http://nbacompare.com/players-like/43/giannis-antetokounmpoThe first, using only box-score data isn't overwhelming. Box score stats don't always show defense.
The second, using player tracking data, includes defensive beasts like Kidd-Gilchrist, Josh Smith, Jimmy Butler, etc.
Oh, and Giannis turns 21 in December. To put that into perspective, Marcus Smart turned 21 in March.