9H is the heavier variant. A normal WAG9 weighs around 123 tonnes but a 9H weighs around 132 tonnes. Apart from that, there's a different application software in WAG9H. Apart from these two differences, they're pretty much the same.
WAG9H has a higher tractive effort compared to WAG9.
Newer WAG9 locos turned out by CLW are all WAG9H locos :)