I think it's hard to answer. At first I thought Ousen was a better strategist when he bested Riboku in the Shukai plains. But later, after Kanki getting close to smashing Riboku, with Riboku saying they might have fought the strongest Qin general, I thought Kanki was indeed the strongest. But then, by Ousen, in a non boasting way, saying he could defeat Riboku 10 out of 10 times, I don't know anymore, Riboku may have underestimated Ousen still when saying Kanki was the strongest Great General. Kanki once said that Ousen always gets what he wants, that he knows what he's doing. At the same token, Ousen was surprised when Kanki used that strategy to ambush the northern Zhao army and kill the general who had killed one of his men, which led him to doing that massacre later on, with Ousen saying --- to the strategy he employed to ambush them --- that even he (Ousen) wouldn't go that far.
So it's tough to say... I'd say they may have specific areas one outperforms the other without a clear stronger one.