Brock tends to shrink back when struck- Haku comes off as legitimately insane from what most wrestlers have said. Now, the "Haku is a bad-ass" thing has produced as many ridiculously over-the-top "Tall Tales" as Andre the Giant's strength and size among the carnies in the wrestling business, but the dude had a reputation for a reason. In a no-holds-barred fight, he'll do the shit you can't even do in UFC- biting, gouging, etc.
In a UFC fight, Brock would win. In a Street Fight? It's probably Haku, even WITH his inflated reputation.