Both are of comparable size and fierce when provoked. Both are armed well; hippos with tusk like teeth and Rhinos with Horns and a armour plated skin. I am no animal expert, but i would call this 50–50.
Push comes to shove, i would put money on the Rhino. They are prbly faster than hippos and the hippo will have some trouble breaking the rhinos hide. Whereas the Rhino might find it easier to put holes in a Hippo.
I wonder if there are any recorded cases of hippos vs Rhinos.