It would be a fairly close battle, which is probably why they almost never clash head to head in the wild. Both animals are highly territorial, but the hippo is much more aggressive. Fights between two male rhinos normally don’t amount to more than some horn clashing and a little urine spraying.
Male hippos, on the other hand, regularly inflict serious injuries on each other with their massive teeth. The rhino has an impressive charge, but in close combat the enormous gape of the hippopotamus’s mouth probably gives it the advantage over the long, unwieldy horn of the rhino.