Id have to go with gorilla. Only because the way gorillas are structured skeletal and muscular wise they have an immense amount of power. Heck a little 100-120 chimp could tear you limb from limb. Also gorillas have got to have a higher ratio of muscle to fat compared to a polar bear who needs that fat to survive.

If we are talking about putting one of the largest gorillas and largest polar bears in an arena then I would have to put all my money on the gorilla...they are tanks