Aminu's biggest advantage over Crowder is his size.
Crowder fights hard, but it's difficult to make a huge impact defensively at the swing position when you're 6'6'' and don't possess fantastic length or athleticism. Iggy's about the same size as Crowder, of course, and he just did it on the biggest stage, but he's a lot more talented.
Crowder's floor is higher than Aminu's, though. Aminu only has great value if he can hit those corner threes consistently.